Some have reported that the 90.0 update has messed with the Menu Bar text colour. In the "Dark" theme, the tabs are black, the Menu Bar is black and the Menu Bar text is black. I can confirm this on Linux Mint 20.1 and Win 10 21H1. I Also confirmed it to be working correctly on ver 89 for both Mint and Win. I also confirmed that the update for ver 89 turned on automatic updates for Firefox (Linux and Win) without notifiying me that it was changing my settings and ver 90 appears to no longer allow user control of updates.

Also, About Mozilla Firefox, Ver 90.0 (64-bit), says it's for Linux Mint 1.0 Is it possible that this is the reason the Menu Bar is messed-up in the Dark Theme?