I have two Desktops, both 22.04 and running the deb version of Thunderbird 115.3.1 (64 bit). Appearance wise both have difference font sizes although in Edit/Settings/General they both have the same settings, sans-serif size 16. Interestingly changing these settings on either machine has absolutely no effect - I'm interested why these two machines have different appearances (as default) and why I cannot change the settings within Thunderbird. The Themes of each Mate machine is different but the Font is the standard ubuntu font and settings.