I can't reproduce that. Does that also happen in Troubleshoot mode (https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode/)?
Type about:config in the address bar > press Enter- click 'Accept the risk and continue' if that appears- search for ui.key.menuAccessKey > change the value to 0Make sure to click the check mark after making the change.
