Have you tried the settings under General>Fonts? Check under Advanced for further tweaking. Your operating system may have other settings but I'm not used to Windows or mac but someone else here surely is.

Hello

Maybe with the help of userChrome.css file, you'll find tutorials on the net.

userChrome.css, to test (and for illustration 17) * {font-size: 17pt !important;}

And userChrome.css does not affect (an example on) about:preferences, then, you have the ability to play with your keyboard Zoom in Zoom out. https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly#w_current-page

Below, an illustration with userChrome.css

"play with your keyboard Zoom in Zoom out" does NOT change the size of the font ( that shrunk overnight) in Bookmarks i created, in the search window, in the numeric zoom percentage. Also why did the font size of the address "support.mozilla.org/en-Us/questions/1502673" become the size of poppy seeds overnight and It will not change when I change the minimum font size in advanced settings.

Found this in another area, so I can't take credit.

Open your Windows Settings Click on Accessibility Click on Text Size Move the Text Size slider over to the right

Running Windows 11 Fire Fox 138.xx

Have a Blessed Day!