Didn’t want to open an issue about this, since this doesn’t seem to be an issue of nixpkgs, but thunderbird itself. Writing here if anyone else also sees the surprise.
After the upgrade, thunderbird refused to open and complained I am running an older version now. Help page on the mozilla side is here.
Since I didn’t do an actual downgrade or something special to this package, the solution was running thunderbird with “–allow-downgrade” flag once. Apparently there needs to be some kind of config version migration.
Similar issue month ago while (1) upgrading Thunderbird and then (2) reverting to an older version. My interpretation is that a Thunderbird database is migrated during Thunderbird upgrade and when reverting, there is no available reverse migration. I ended up re-configuring from scratch my mail accounts.
Does someone know how NixOS deals with upgrades followed by downgrades of apps relying on stateful databases ? How database migrations are handled ?
Specifically for the case of thunderbird the application is left to manage state itself so you would have required a backup of your ~/.thunderbird directory before upgrading as the effect of upgrading on state is permanent (unless thunderbird itself had a mechanism for rollback).