It just occured to me that if you want to use Ubuntu without snap, you could uninstall the snap package itself (I’m not on Ubuntu, so you might need to find it), then put a ‘hold’ on the package to prevent it being reinstalled. That should, in turn, prevent any package versions that use snap from being installed.

Initially uninstalling snap might require removing any packages that use it, but that’ll tell you what you need non-snap versions of.

I suspect that what’s happened is you installed the apt version, then at some point upgraded it and there was a version in the main repo that had a higher version number and installed the snap version. If two repositories both have a package with the same name, and no other rules in place, the higher version number wins.

If that is the case, you need to pin the firefox package to the mozilla repository. You can find more details here: https://wiki.debian.org/AptConfiguration