Hi FFv48usr, I do not want to suggest to have two versions of Firefox installed, however that is the first temporary work around I can think of.

Is it also possible to contact the add on developer to sign the add on for Firefox 48 compatibility. MDN Addon Distribution

How do I install a previous version to make the app work while still keeping v48 (and subsequent updated versions) as active when I use FF?

Is the problem that the extension was never signed by Mozilla?

That requirement was first applied in Firefox 43 and there was a way to make an exception until Firefox 47. The exception preference is no longer available starting in Firefox 48.

The exception preference is still available in the "Developer Edition" of Firefox, which is a test version that probably is less stable than you'd like.

The exception preference also is still available in the Extended Support Release of Firefox 45. Although the ESR version is intended for business customers, you can run it if you like. ESR doesn't get new features, but it gets most security fixes.

You can find more information on this page: https://www.mozilla.org/firefox/organizations/

I suggest making a "just in case" backup before using the ESR installer. This article has tips on that: Back up and restore information in Firefox profiles.

When you view the full list of installers, you'll notice there are a Windows and a Windows 64-bit version. The 64-bit version is highly restricted on what plugins you can use, only Flash and Silverlight, no Java, no Adobe PDF, etc. For most users, the regular Windows version is the best choice.