Mica transparency effect

[Dekrate]

I think that both Thunderbird and Firefox should have an option to enable Mica transparency effect on computers with Windows 11 installed.

[oyrslyng]

With the release of the Nova redesign, mica is seemingly broken. Enabling the flag gives a very mild mica effect on top of the new colors, but also turns the address bar white/grey when in dark mode. Enabling `browser.theme.native-theme` fixes this and makes the colours disappear, but adds a 1px white line above the whole toolbar if horizontal tabs is used (this white line appears without nova enabled as well).