Did you create a new profile when you switched the versions ?

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles
• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

This is a link to the built-in fallback Widevine CDM files that redirect to the actual file.

• https://searchfox.org/mozilla-release/source/toolkit/content/gmp-sources/widevinecdm.json

media.gmp-widevinecdm.enabled => true
media.gmp-widevinecdm.visible => true
media.gmp-widevinecdm.version => needs to match installed Widevine version in gmp-widevinecdm/xxx

These links were very helpful. I was able to recover history, bookmarks, etc using details from the "recovering important data" link. I do wish there was an easier way to recover extensions from old to new profiles. I read about FEBE and MozBack (sp?) and they both appear to be outdated. I was able to manually copy the extensions folders and extension json files from old to new profile. Then in the new profile I disable and enabled each of the extensions which seemed to allow them to be "seen" and used by Firefox. Not sure where the properties for some of the extensions are stored but I'll admit laziness here and slowly recreate the settings as I go. (NoScript and UBlock for instance)

Do you have a method to recover extensions / add-ons short of re-installing them? Is there a default path where extensions store their settings by default I might check?

Extension settings are stored with unique identifiers that only work in that profile. Often there is an import/export feature for those that have many settings.