Firefox 70 for developers
=========================

This article provides information about the changes in Firefox 70 that will affect developers. Firefox 70 was released on October 22, 2019.

Changes for web developers
--------------------------

### Developer tools

#### Debugger updates

* In the Debugger you can now set breakpoints for DOM Mutation, so execution will pause when a node or its attributes are changed or when a node is removed from the DOM (Firefox bug 1576219).
* The Debugger now shows an overlay on the page when it is paused, with basic stepping buttons to let you step and continue (Firefox bug 1574646).
* The Debugger now shows sources that already got discarded by the engine (usually scripts that execute once during page load), so you can properly set breakpoints to debug when they execute next (Firefox bug 1572280).
* The Debugger's scopes panel grouping has been simplified, consolidating additional scopes previously shown above the top level function (e.g. blocks created by `let`, `with`, or `if`/`else`) (Firefox bug 1448166)
* The Debugger now retains the currently selected and expanded variables in the scopes panel while stepping (Firefox bug 1405402).
* The Debugger now handles stepping over async functions correctly, making asynchronous function debugging easier (Firefox bug 1570178).
* When debugging in Container sessions (useful for testing different logins), the sources in the debugger are now shown correctly (Firefox bug 1375036).
* `debugger` statements can be now disabled in the Debugger by setting a breakpoint on them and switching the breakpoints to "Never pause here" (Firefox bug 925269).
* WebExtensions developers can inspect `browser.storage.local` from the Extension Storage item under the Storage tab (Firefox bug 1585499).

#### Other updates

* An icon will be displayed next to inactive CSS properties in the Rules view of the Page Inspector, which you can hover over to get information on why it is inactive (Firefox bug 1306054).
* In the CSS Rules view, the color picker on foreground colors now tells you whether its contrast with the background color meets accessibility conformance criteria (Firefox bug 1478156).
* The Accessibility inspector's Check for issues dropdown now includes keyboard accessibility checks (Firefox bug 1564968).
### HTML

* Firefox can now suggest securely-generated passwords to the user in the following situations:
    + An `<input>` element has the `autocomplete="new-password"` attribute value.
    + The user opens the context menu on any password input element, even if it is not intended for new passwords.
### CSS

* Opacity values like for `opacity` or `stop-opacity` can now be percentages (Firefox bug 1562086).
* `grid-auto-columns` and `grid-auto-rows` now accept multiple track-size values (Firefox bug 1339672).
* A number of text-related CSS properties have been enabled by default (Firefox bug 1573631):
    + `text-decoration-thickness`.
    + `text-underline-offset`.
    + `text-decoration-skip-ink`. The default value is `auto`, which means that by default underlines and overlines are now interrupted where they would otherwise cross over a glyph.
* The `display` property now accepts two keyword values representing the inner and outer display type (Firefox bug 1038294, Webkit bug 1105868 and Webkit bug 1557825).
* The `font-size` property now accepts the new keyword value `xxx-large`. (Firefox bug 1553545).
* The `:visited` pseudo-class no longer matches `<link>` elements, for logic and performance reasons (Firefox bug 1572246; see Intent to ship: Make <link> elements always unvisited and [selectors] :link and <link> for more reasoning as to why).
* We now support an `auto` value for the `quotes` property (Firefox bug 1421938).
* Stylesheets contained in `<style>` elements are now cached for reuse, to improve performance (Firefox bug 1480146). Note that this currently doesn't include stylesheets that contain `@import` rules.
* The `<ratio>` type now accepts `<number>/<number>` or a single `<number>` as a value. (Firefox bug 1565562).

#### Removals

* We have retired support for 3-valued <position> (excluding background)(Firefox bug 1559276).
* The `none` value is now invalid in `counter()` / `counters()` — a change which makes the Level 3 spec match CSS 2.1 Firefox bug 1576821).
### SVG

* Cut, copy, and paste events are now dispatched to SVG graphics elements (Firefox bug 1569474).
### MathML

* The deprecated `mode` attribute on `<math>` elements has been removed (Firefox bug 1573438).
* Non-zero unitless length values, such as `5` for `500%`, are no longer supported.
* Length values ending with a dot, such as `2.` or `34.px`, are also unsupported now.
### JavaScript

* Numeric separators are now supported (Firefox bug 1435818).
* The `Intl.RelativeTimeFormat.formatToParts()` method has been implemented (Firefox bug 1473229).
* The `BigInt.prototype.toLocaleString()` method has been updated to work with the `locales` and `options` parameters per the ECMAScript 402 Intl API. Also, `Intl.NumberFormat.format()` and `Intl.NumberFormat.formatToParts()` now accept `BigInt` values (Firefox bug 1543677).
* Per the latest ECMAScript specification, a leading zero is now never allowed for BigInt literals, making `08n` and `09n` invalid similar to the existing error when legacy octal numbers like `07n` are used. Always use a leading zero with the letter "o" (lowercase or uppercase) for octal `BigInt` numbers (i.e. `0o755n` instead of `0755n`). See Firefox bug 1568619.
* The Unicode extension key "nu" is now supported for the `Intl.RelativeTimeFormat` constructor and the `Intl.RelativeTimeFormat.resolvedOptions()` method now also returns `numberingSystem` (Firefox bug 1521819).
### APIs

#### DOM

* The `back()`, `forward()`, and `go()` methods are now asynchronous. Add a listener to the `popstate` event to get notification that navigation has completed Firefox bug 1563587.
* We've added support `DOMMatrix`, `DOMPoint`, etc. in web workers (Firefox bug 1420580).
* A few more members have been moved from `HTMLDocument` to `Document`, including `Document.all`, `Document.clear`, `Document.captureEvents`, and `Document.clearEvents` (Firefox bug 1558570, Firefox bug 1558571).
* Notification permission can no longer be requested from inside a cross-origin `<iframe>` (Firefox bug 1560741).

#### Media, Web Audio, and WebRTC

* The `RTCPeerConnection.restartIce()` method has been added. This is one of the four changes needed to implement the new "perfect negotiation" mechanism; the rest will come in future Firefox updates (Firefox bug 1551316).
* The `RTCPeerConnection.setRemoteDescription()` method can now be called with no parameters. This is another "perfect negotiation" update (Firefox bug 1568292).
* `MediaTrackSupportedConstraints.groupId` is now supported, and returns `true` since the `MediaTrackConstraints.groupId` property is now supported (Firefox bug 1561254).
* Several new Web Audio API features have been implemented/updated:
    + `AudioContext.getOutputTimestamp()` implemented (Firefox bug 1324545).
    + `AudioContext.baseLatency` and `AudioContext.outputLatency` implemented (Firefox bug 1324552).
    + `MediaElementAudioSourceNode.mediaElement` and `MediaStreamAudioSourceNode.mediaStream` implemented (Firefox bug 1350973).
    + The `ChannelMergerNode()` constructor now throws errors if you try to set `channelCount` and `channelCountMode` to invalid values (Firefox bug 1456263).

#### Canvas and WebGL

* We now support `CanvasRenderingContext2D.getTransform()`, and the newer variant of `CanvasRenderingContext2D.setTransform()` that accepts a matrix object as a parameter rather than multiple parameters representing the individual components of the matrix (Firefox bug 928150).
### HTTP

* The default referrer policy for third-party tracking resources is now `strict-origin-when-cross-origin` when Enhanced Tracking Protection is turned on (Firefox bug 1569996).
* The size of the `Referer` request header is now limited to 4 KB (4,096 bytes). If an overly long referer exceeds the defined limit, only the origin part will be sent (Firefox bug 1557346).
* The HTTP cache is now partitioned per the top-level document's origin (Firefox bug 1536058).

#### Removals

* The `X-Frame-Options` `allow-from uri` directive has been removed. Use the `Content-Security-Policy` header with the `frame-ancestors` directive instead (Firefox bug 1301529).
### WebDriver conformance (Marionette)

* Updated the `WebDriver:TakeScreenshot` command to be Fission compatible. It means that content from cross-origin iframes is now included in a page's screenshot. Or when using it from chrome scope that the active tab's content is visible now inside the browser window. (Firefox bug 1559592).
* `WebDriver:TakeScreenshot` no longer accepts a list of DOM elements as used for highlighting (Firefox bug 1575511).
* `WebDriver:ExecuteScript` and `WebDriver:ExecuteAsyncScript` no longer sets `window.onunload` in ways that are web-exposed (Firefox bug 1568991).
Changes for add-on developers
-----------------------------

### API changes

* Added a new parameter to the `topSites.get()` method that causes the method to return the list of pages that appear when the user opens a new tab (Firefox bug 1568617).
* The `privacy.network` property's `WebRTCIPHandlingPolicy` sub-property's permitted values have been amended (in Firefox bug 1452713) to match the behavior seen in Chrome as follows:
    + `disable_non_proxied_udp` previously prevented the use of WebRTC if no proxy was configured. Now a proxy is always used if one is configured, but otherwise a non-proxied connection is permitted.
    + `proxy_only` can be used to provide the old behavior; this has the effect of only allowing ICE negotiation over TURN on TCP using a proxy; no other connections are allowed.
### Manifest changes

#### Removals

The following theme key properties, which provided aliases for theme keys used in chromium-based browsers, were removed:

* `images` property `headerURL`, themes should now use `theme_frame`.
* `colors` properties:
    + `accentcolor`, themes should now use `frame`.
    + `textcolor`, themes should now use `tab_background_text`.
See also
--------

* Hacks release post: Firefox 70 — a bountiful release for all
Older versions
--------------

* Firefox 69 for developers
* Firefox 68 for developers
* Firefox 67 for developers
* Firefox 66 for developers
* Firefox 65 for developers
* Firefox 64 for developers
* Firefox 63 for developers
* Firefox 62 for developers
* Firefox 61 for developers
* Firefox 60 for developers
* Firefox 59 for developers
* Firefox 58 for developers
* Firefox 57 for developers
* Firefox 56 for developers
* Firefox 55 for developers
* Firefox 54 for developers
* Firefox 53 for developers
* Firefox 52 for developers
* Firefox 51 for developers
* Firefox 50 for developers
* Firefox 49 for developers
* Firefox 48 for developers
* Firefox 47 for developers
* Firefox 46 for developers
* Firefox 45 for developers
* Firefox 44 for developers
* Firefox 43 for developers
* Firefox 42 for developers
* Firefox 41 for developers
* Firefox 40 for developers
* Firefox 39 for developers