Does it still happen in Troubleshoot Mode?

Is this with the Developer Edition or with the Beta 120 release ?

Note that PB mode may use a dark theme.

• about:config => browser.theme.dark-private-windows

Does it still happen in Troubleshoot Mode?

No. I've had [4 extensions](https://i.stack.imgur.com/4Rg0Q.png) disabled all, problem persists. Not private browsing. Short of reinstalling, any other steps?

Firefox Developer Edition, 120.0b2 (64-bit)

Try changing your theme to the default Light theme.

Go to about:config and make sure toolkit.legacyUserProfileCustomizations.stylesheets is default false.

If by default you mean this, that's what I've been using, yes. That about:config setting is false.