Hi, this has to do with your GTK+ theme as Firefox will be themed by your chosen theme respectively, try some different themes in Mint by going to Appearance, or find the customization options in Settings. If this helped solve your problem, please mark as solution

Thanks for that quick reply! It does narrow it down a bit. What I mean is, comparing themes on this PC to the other Linux PC (where it works) shows that they're both using the same theme settings. BUT, on the PC where it's not working if I change the theme for "Controls" to Mint-Y-Dark, it works! If I change the theme for Controls to anything else, like Mint-X options and even other Mint-Y options, it doesn't.

You might need to re-install your version of Mint, perhaps during an upgrade, the theme configuration files got corrupted -- during upgrades, you should update the lists when prompted instead of pressing keep. If you don't want to reinstall your copy of Mint, you could keep the Window controls on Mint-Y-Dark if it continues to work for you.

If this solved your problem, please mark as solution.

I don't know. The last 2 Mint versions were done as clean installs, not in-place upgrades. That would mean both times the Mint theme config files were corrupted? And it only happened with Firefox after the big rebuild Mozilla did.

Fingers crossed that this is fixed in a future update to either Firefox or Mint.

Oddly enough, the background-color hover effect works in the Menu bar drop-downs of Firefox. Just not the Page Actions drop-down or Toolbar menu drop-downs.

If you change the GTK3 theme while Firefox was still running you may need to restart Firefox for the theme to fully work on Firefox.

The official Firefox Releases since 46.0 have required GTK 3.4 (three.four) at minimum installed to run and also needed GTKL3 themes to rtheme Firefox as GTK2 themes will not work. Some Linux distros have delayed this change in their provided Firefox packages so it is Not a recent change by Mozilla per say.