Here are your choices; 1) Contact its support and ask if they will update 2) Find a replacement 3) Downgrade to a prior Firefox 4) Do without

Your files should still be there but inaccessable because the addon that was using them does not work anymore. In order to access them you can install and use the Firefox ESR version available here :

https://www.mozilla.org/en-US/firefox/organizations/all/

That way you can either use this version to do your work or at the very least you can access your sessions and maybe save them using the Bookmark All Tabs option which creates a bookmark folder where you'll find all tabs that were in that session. You can restore the entire session by right clicking that folder and selecting Open All in Tabs. I know it's not the same, but it's a functional method.

P.S. : Firefox and Firefox ESR use the same files and addons so changing something using one of them changes it for the both of them. This is also the reason why they cannot both be executed and running at the same time.

@Alex Thanks for the info and link. I've found the Session Manager session files in C:\Users\<user name>\AppData\Roaming\Mozilla\Firefox\Profiles\<random-alpha-numeric>.default\sessions\

That's a good start (I had been looking in 'local' instead of 'roaming').

Simply checking a setting to restore a previous session is the best way, but this at least works: Click the hamburger icon on the right end of the main toolbar.

  Select Restore Previous Session  Hope that helps a bit.