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I’m currently not using FF, so I won’t give you any specific solution.
Avast should work correctly with FF, at least in most cases.
A general suggestion would be to uninstall FF, reboot, clean any remnants, reboot, install the latest STABLE version of FF again, reboot and test. Please be sure to set FF as your default browser.
There is probably some simpler solution (like some file extension assignment or alike), but you would need someone currently using FF and latest avast to test it and help you.
If you try that general suggestion, please report back. Or wait for some other more specific and useful suggestion.