Sorry to hear about this problem.

If you haven't already, could you close out of Firefox and do a normal Windows shutdown and restart to complete any pending updates and release any file locks.

If Firefox again starts up without your session, could you check whether Firefox might have switched profiles? Here's how:

Profile Manager Page

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least one profile and could list many. (Each profile has two folders and one or more buttons. Do not use any Remove buttons!)

The profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

Do you have another profile named default or default-longnumber listed on this page? If so, you can check whether it is the one you were using recently by clicking its Launch profile in new browser button.

If it isn't what you want, simply close that new window.

If it IS what you want, back on the about:profiles page, click the Set as default profile button for that profile so Firefox uses it automatically at the next startup.

Any progress so far, or do we need to keep looking for your data in some other places?

It was the profiles. I switched to default and the previous session opened. Thanks for your help!