STR:

1. Press control+l to focus the Location bar.
2. Tab to the group of toolbar buttons containing the Firefox Accounts button and then press right arrow until Firefox Accounts is focused.
3. Press space to activate it.
   • Expected: The first button inside the Firefox Accounts panel should get focus, as is the case for the main Firefox menu button, the Library button, etc.
   • Actual: Focus goes nowhere.

Bug 1536514 fixed things so that this button can be activated from the keyboard. However, focus still goes to limbo when it is pressed. We apparently don't want focus to go somewhere for mouse users, but for keyboard users (especially screen reader users), focus shouldn't go to limbo.

This should be a trivial fix. In gSync.toggleAccountPanel, we should just be able to pass the event to PanelUI.showSubView:
https://searchfox.org/mozilla-central/source/browser/base/content/browser-sync.js#292
That is, this should be:
PanelUI.showSubView(viewId, anchor, aEvent);
We should also add a test to browser/base/content/test/keyboard/browser_toolbarButtonKeyPress.js.

https://hg.mozilla.org/mozilla-central/rev/5fcf07f6cc74

Jamie, can you verify this is working correctly now?

Verified fixed in Firefox 68.0a1 build 20190324214519.

https://hg.mozilla.org/releases/mozilla-beta/rev/cfb2d7610755