What you need to do is download and disconnect from the internet so it doesn't up and maybe go and uncheck for updates. That's my only guess but then again using out-dated Browser is a security hole and not really advised here.

Installing multiple versions on Mac OS X is basically dragging the Firefox application out of the DMG file to the desktop and rename the folder. You can drag the renamed Firefox application folder to the Applications folder like you would normally install Firefox. You can possibly eject the DMG file and and trash the installer.

You need to use the terminal to create a new profile for that version. Firefox also comes with the about:profiles page that has a button to create a new profile. Make sure that no profile is selected as Default=1 to make the Profile Manager show when you start Firefox. You can check that in the profiles.ini file.

• 1. SK#/profile-manager-create-remove-switch-firefox-profiles

from https://support.mozilla.org/en-US/kb/end-support-adobe-flash

Firefox version 84 was the final version to support Flash. Firefox version 85 (released on January 26, 2021) shipped without Flash support, improving our performance and security. There is no setting to re-enable Flash support. Even if you install an older version of Firefox, the Flash plugin itself stopped loading Flash content as of January 12, 2021. See Adobe’s Flash Player End of Life information page for more details.

I need to run a page that uses Adobe Flash

The site has not been updated to use HTML5 or such since? The Adobe discontinuing Flash was rather old news announced way back in July 2017.