https://support.mozilla.org/en-US/questions/1335097 might be of use to you too, for what it's worth. (essentially: if you set about:config pref `toolkit.legacyUserProfileCustomizations.stylesheets` to `true`, then you can create a file `chrome/userChrome.css` inside of your Firefox profile with arbitrary CSS customizations. E.g. if I add `* { font-family:monospace !important }` there, then all of the Firefox UI renders with my default monospace font. This might be the tool you're looking for? Though if there really is some way to change the system font in a way that other apps respect but Firefox does not, it'd be good to know more about it.)
Bug 1822208 Comment 5 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
https://support.mozilla.org/en-US/questions/1335097 might be of use to you too, for what it's worth. (essentially: if you set about:config pref `toolkit.legacyUserProfileCustomizations.stylesheets` to `true`, then you can create a file `chrome/userChrome.css` inside of your Firefox profile folder, with arbitrary CSS customizations. E.g. if I create that `chrome` folder in my profile, and create a new file `userChrome.css` in that folder which just contains `* { font-family:monospace !important }` there, then all of the Firefox UI renders with my default monospace font. This might be the tool you're looking for? Though if there really is some way to change the system font in a way that other apps respect but Firefox does not, it'd be good to know more about it.)
https://support.mozilla.org/en-US/questions/1335097 might be of use to you too, for what it's worth. (essentially: if you set about:config pref `toolkit.legacyUserProfileCustomizations.stylesheets` to `true`, then you can create a file `chrome/userChrome.css` inside of your Firefox profile folder, with arbitrary CSS customizations. E.g. if I create that `chrome` folder in my profile, and create a new file `userChrome.css` in that folder which just contains `* { font-family:monospace !important }` there, then all of the Firefox UI renders with my default monospace font. This might be the tool you're looking for? Though also: if you really have found some unrelated way to change the system font in a way that other apps respect but Firefox does not, it'd be good to know more about it.)