Comment 6 for bug 1697800

None of the upgrade tools are in a position to check architecture features before installing a package. There may not be a happy solution here.

Once you downgrade to a working version of Firefox you can use

sudo apt-mark hold firefox

to try to keep the updating tools from replacing it.

Note that doing this will prevent you from getting security updates for Firefox.

To undo this operation you can run

sudo apt-mark unhold firefox

Do not undertake this step lightly -- running a known-insecure webbrowser is probably a bad idea.

Thanks