I use Manjaro Stable with KDE, on a Lenovo T480s with an external monitor (that has a significantly higher resolution).
Thunderbird seems to have now decided that the minimum horizontal size will be significantly larger than my laptop screen, even when my external monitor is disconnected. My wild guess is that this is some percentage of the resolution of my external monitor. When I maximize Thunderbird on my laptop part of the window disappears, in the example below you see the list of accounts are too far on the left side. You also see here the relative difference in screen size (the screenshot of leaves on top is the external monitor).
I assume that deleting all configs for Thunderbird will probably resolve the issue, but I would prefer not having to do so (it takes time to reconfigure and honestly in my mind that should be a last resort solution to an application which is considered stable).
I also have Thunderbird beta installed, and do not experience the issue there.