Hi, I'm using Firefox 90 browser on Ubuntu 21.04. Somehow the UI font for my profile changed from the default and I can't find any way to reset it (see bad_ui_font.png) My tabs, menus and preferences all got set to some fixed-width font, and I can find anywhere in the preferences or about:config, or in my profile folder where I can change it back. If I create a new profile, then the UI font is fine (see good_ui_font.png). But as soon as I login to my firefox account and sync, then the new profile gets the bad UI font. I don't want to have to recreate my profile from scratch and lose all my settings just to correct the UI font. But this is very annoying. How can I restore the default UI font?

Thanks.