Seems to be a problem with themes. If I turn themes off, the buttons appear.

Another Windows 7 user mentioned that animated themes have this problem, but they are able to use other themes.

I use Windows 7 and the maximize, minimize and close buttons disappear for me as well, when I use the Firefox Alpenglow theme that comes with Firefox or any added theme I've tried in Firefox 90, including "Solid" themes like Proton Colors, Polynial design, Classic Blue and Dark Blue. The problem disappears when I switch to the System, Light, and Dark themes that come with Firefox.

I was able to solve ^{work around} the problem by switching my Windows theme in Control Panel -> Personalization. I had been using an Aero theme. After changing to a Basic Windows 7 theme, all of my Firefox 90 themes are working.

Not really a solution as I prefer Windows Aero themes. (I tried disabling transparency but it didn't help.)

Any real solution or fix for this problem?

. . . the maximize, minimize and close buttons disappear for me as well, when . . .

Have you filed a bug report?

See also:

• 1715944 - [win7] Changing OS theme with Firefox opened displays a grey bar over browsers controls when changed to Aero theme or black bar near buttons for other themes

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

https://bugzilla.mozilla.org/buglist.cgi?query_format=specific&order=changeddate%20desc%2Cbug_id%20desc&chfieldto=Now&chfieldfrom=2021-01-01&content=Aero

AliceWyman said

. . . the maximize, minimize and close buttons disappear for me as well, when . . .

Have you filed a bug report?

No, I didn't.

By the way, I decided to keep using a Basic Windows 7 theme, which doesn't have the "missing maximize, minimize and close buttons" bug with added themes. All windows open faster now with the Basic Windows 7 theme compared to Aero, even when opening other applications like SeaMonkey, which used to take a long time to load.

I decided to keep using a Basic Windows 7 theme, which doesn't have the "missing maximize, minimize and close buttons" bug with added themes. All windows open faster now with the Basic Windows 7 theme compared to Aero

Interesting.