after the last update, the themes are covering up the minimize, maximize, and close buttons. the buttons are still there, you just can't see them while a theme is active.
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This came up before. See https://support.mozilla.org/en-US/questions/1344224
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 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.