After using gnome-tweaks to change my system fonts

Sounds pretty straight forward. Tweaks is the issue and not the browser. If the tweak doesn't affect all user, change to another account and see what happens.

Made a new user account and logged into cinnamon. No issue there, so it is specific to my user. Tried resetting to default in gnome-tweaks but that didn't change anything. Tried reloading font cache. Tried flatpak version. Tried disabling hardware rendering. Tried changing firefox's default fonts. Nothing worked.

Like you said, After using gnome-tweaks is when the issue started. All the hoop jumping in Firefox is not the issue. Just try another user account in your Gnome desktop and see if it has the same issues. Again... not a Firefox issue, you tweaked and now it's broken.  ;-))