I had to uninstall Firefox (I used CleanMyMac 2), installed the latest v103.0, and found the font in the bookmarks sidebar too small. To change the size, enter “about:config” (without the quotes), click through the warning, search for “systemfont”, and edit. The default is 100. I changed it to 110. Quit and relaunch Firefox to see the change.
A somewhat easier way to do this is going into Preferences, scrolling down to Fonts in the General section. Bonus: you can also change the default font (used for plain text and unstyled content), and clicking the Advanced button allows you to fine-tune which system fonts are used for Proportional, Serif, San-Serif and Fixed typefaces, and specify minimum font size.
The Fonts settings under Preferences affect web page rendering, but not the font size used for bookmarks in the sidebar. That’s the system font internal to Firefox itself, which I needed to change.
Ah! My apologies for misreading your original post. To be honest, I wasn’t even aware that Firefox had the ability to change the font size used in the UI — Vivaldi, which I was using previously, has this option available in its Preferences and is one of the features I miss most.
No worries. Mozilla didn’t make it easy to find. :)
One thing to note: setting the system font size via about:config as described above also affects any Zoom setting you’ve set in Preferences, so may need to adjust the latter downwards to avoid oversize text on web pages.