Debian is on Firefox 78, which is an official long-term support version (Extended Support Release in Mozilla-speak). The point of those LTS versions is exactly to only contain critical bug fixes and no other changes.
Debian stable you mean. I am talking about unstable.
https://tracker.debian.org/pkg/firefox says that Debian Unstable should have version 90 as of last month. If you're not getting updates, perhaps the mirror you're using is stale?
Debian unstable isn't really meant to be a fully fledged desktop OS on its own. I mean, yes, it can be. But there are problems like the one you mentioned - around freeze time for the next release the entire update policy dramatically slows down.
Personally I'm a big fan of Fedora.
I will be moving to nixos.
For unstable, you should currently take Firefox from experimental. I assume, like for previous releases, it will be here by tomorrow. Due to the freeze for Bullseye release, the dependencies to put it in unstable are not present.
Aren't the firefox upgrades largely in-browser now? I haven't seen firefox even upgraded in the repos but two clicks in Firefox' menu got me the last version.
I am not aware of any distribution that does not disable the in-firefox updates.