Since you're blocking Mozilla domains, my first thought was that it might have to do with the automatic malware checks for downloaded files. But the knowledge base article says it only checks executables, and it doesn't sound like it tries to contact a Mozilla server either.
But yeah, maybe you want to try turning that off in the settings for debugging either way.
The other suspicion is immediately Snap, primarily because I've seen quite some brokenness from Snap Firefox already.
You can try downloading the non-Snap version from the webpage directly: https://www.mozilla.org/en-GB/firefox/all/desktop-release/linux64/
That'll give you a .tar.bz2, which you just unpack and run the firefox binary inside.
If it works there and you want to permanently switch, you probably want to use Mozilla's APT repo: https://support.mozilla.org/en-US/kb/install-firefox-linux#w_install-firefox-deb-package-for-debian-based-distributions-recommended