Upon further inspection it seems what happened is that all my un-pinned tabs were moved to a separate window and then that window was closed. I was able to get the tabs back restoring the closed window (History -> Recently Closed Windows). What is going on with this crazy behavior in Firefox?

It is not very likely that the pinned tabs were moved to another window. More likely is that you opened a second window and continued to work in the first window. Then you closed the first window with the pinned tabs and then possibly closed the second window, but left Firefox running as is possible on Mac.

On Mac closing Firefox can be more complicated because you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc.

Make sure to use "Quit Firefox" with all windows still open, especially if you want to use "Restore Previous Session" or have pinned tabs in a window. If you lost important windows then check "History -> Restore Previous Session" and "History -> Recently Closed Tabs/Windows".

I think the response from cor-el is close to what happened, although it is still unclear. For years on this computer I had 2 windows open in Firefox with many tabs in each. This caused problems and confusion so I wanted to consolidate all the tabs into one window. I had just done that and had closed the 2nd window. That is when all the tabs in my one and only window suddenly disappeared. Anyway, I was able to get them back, as I mentioned, with "History -> Recently Closed Windows". But now I have 2 windows again. So I will manually combine all the tabs back into one window then immediately quite Firefox to hopefully lock in this setting.

By the way I never see "History -> Restore Previous Session". When is this visible?

Make sure to use "Quit Firefox" with all windows still open, especially if you want to use "Restore Previous Session". Otherwise, check "History -> Restore Previous Session" and "History -> Recently Closed Tabs/Windows".

Hope This Works, Peter

Today I lost tabs again - this time all of my tabs. This is the third time in the last 3 months that I have lost some or all of my tabs. This time it happened when I updated macOS and the computer was restarted as part of that process.

Each time this happens I can't recover the tabs from within FF. I have to restore the profile folder from a recent backup.

I don't ever willfully open more than one FF window (sometimes websites manage to do this). Also, I never use the Close Window command. I only close tabs or quite the app all together. But in this case FF was shutdown by the OS as part of the update process.