Thunderbird's all-important "Quick Filter" tab has stopped working under recent updates to Horus (i.e. it used to work just fine under Horus). More specifically, clicking on the "Quick Filter" tab just causes it to turn grey, but the search bar does not appear underneath it, and so it cannot be used.
Although Thunderbird is an unsupported app, this sudden loss of functionality is almost certainly due to one of the many recent improvements to Elementary's "gala" window manager. It is therefore likely that it affects other programs as well.
I've tried both the flatpak and apt repository versions (both of which are 102.10.0), and it makes no difference. So I hope someone at Elementary will look into it (it's probably just a refresh problem of some kind, perhaps in Thunderbird's css style sheet, but that's beyond my paygrade).