Firefox and Thunderbird can't read /usr to add apps to handlers file
In some recent update I lost all the application handlers in both Firefox and Thunderbird. When I search for "other apps" it reports no permission to access /usr. The directory has 755 permissions, however.
I have the same issue with Firefox in Ubuntu 21.10 (can't really read usr/ even though the permissions are fine).
As stated in the comments, the problem is the snap.
So remove the Firefox snap and re-install it via apt.
sudo snap remove firefox
sudo apt install firefox
(It remains a mystery to me why Ubuntu has Firefox snap by default, since it is flawed)