Much of the need for the theme font size add-on appears to stem from Thunderbird not correctly re establishing scaling when a new monitor/video card appears. This often appears as a side issue for folk migrating their profile to new hardware.
If a new profile is created the user interface appears to scale differently (larger size on the new hardware) to if the old user profile is used. I am looking for a file we can rename, or a setting we can reset to default to force the software to basically start over in the way it would with a new profile.