Affected versions

• Fx 86.0b8
• Fx 87.0a1

Affected platforms

• Windows 8.1 x64
• Windows 10 x64
• Ubuntu 16.04 x86

Steps to reproduce

1. Launch Firefox with a clean profile.
2. Access a webpage (ex. https://www.reddit.com/) in order to push a notification icon in Firefox address bar.
3. Click the Notification icon from address bar, the notification prompt shows and navigate to the close button with Tab.

Expected result

• The close button selection is correct.

Actual result

• The close button has a strange selection, it is surrounded with a black border.

Regression range

• First bad: 2020-11-17 (build ID = 20201117215529)
• Last good: 2020-11-16 (build ID = 20201116210217)
• Pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b9dd8e54c8ae449638635758a6571285655a4d0c&tochange=9dd0b13d77b96a7e1fd6b48c23f5fed1363498d2
• Potential Regressor: https://bugzilla.mozilla.org/show_bug.cgi?id=1677376

I don't se the close button on Nightly?

The close button on notification dropdown it looks strange, see attached close-button.png. Reproduced on latest nightly Firefox 88.0a1 build, on Ubuntu 18.04 x64, Windows 10 x64 and macOS 10.15.
Note that is reproducible with both pref widget.non-native-theme.enabled = true and false too.
Not reproduced on beta 87.0b7.

I fixed that in bug 1695323.