Merge branch 'develop'
Updated README.md
FF4 -> FF20
Mixed Content Blocker doorhanger icons
(Web) Notification and pointer lock icons
Paint Flashing icon
MPL 2.0 licenses
Updated about:newaddon (added max size to the add-on icon, padding etc)
Updated DevTools (FF23+)
New Tab thumbnail undo feature
Added more FF23-related DevTools icons and files
Added #tosDesc to syncSetup.css
Removed backwards-compatible (FF4-18) :-moz-placeholder code
Forced 100% resizer background-size
Changed .downloadTypeIcon height/width
Updated tabview.png
Added slow startup turtle icon
Updated volumeThumb.png
Updated downloads.css
Updated RSS subscribe UI-related files
Removed quotation marks