Has been happening to me for a couple of years - multiple IMAP google accounts. Mostly, it happens when I delete a bunch of stuff from the Archives folders. Thunderbird would freeze. Using Wayland, after Thunderbird crashes, Wayland UI was frozen and I had to reboot. So, went back to X (xfce4); Thunderbird crashing does not kill X. Recently, if I leave Thunderbird alone, it eventually comes back.
RME