Bug 1105857 - [Messages] User doesn't receive any notification about incoming message if native modal dialog is shown. r=schung

Sure, but just to confirm @rnowm :)

I'll use the following to support RTL properly:
```
form[role="dialog"][data-type="confirm"] menu button {
  margin: 0;
  -moz-margin-end: 1rem;
}

form[role="dialog"][data-type="confirm"] menu button:last-child {
  -moz-margin-end: 0;
}
```

And previously we have 1.5rem distance between buttons (right 0.5 margin from first button and left 1rem margin from the second one) and additional 0.5 margin from left and right. So you're intentionally want to decrease distance between buttons to 1rem, and remove additional 0.5rem left margin from first button and 0.5rem right marging from second one? :)