Hi,
I didn’t tried Keepassxc from Snap, because I never used the proxy-script and my intention was to exclude possible unrelated errors.
I use AppImage Keepassxc for a long time with no problems with Firefox from deb, since jammy Firefox-ESR. I downloaded yesterday Firefox as tar.gz, excluded it and test it. No problem with Keepassxc from AppImage. The manifest is correct, points to /home/keywan/.local/bin/keepassxc. I copied the AppImage to my home and tried that, but still doesn’t work.
I installed Keepassxc 2.7.1 from PPA and it worked with Firefox-Snap and Firefox-ESR. Although Keepassxc from Flathub worked without any problem with Firefox ESR or downloaded tar.gz.
Maybe it is a AppImage specific problem?!
Let me know, if I can help and should test anything.