Did this problem start before, during, or after you installed Firefox 87?

Firefox 87 should not complain as it is the latest version, it should "up convert" older profiles happily.

Sometimes users end up in an awkward situation where two different version of Firefox are installed, one that runs from a desktop shortcut and one that Windows uses when you click a external link or shortcut outside of Firefox. Have you noticed a difference in the behavior in those two different ways of starting Firefox?

My first "computer" as an IBM Mainframe in about 1965. I managed Data Centers of Mainframes from 1965 until I retired in ~ 2009 from IBM. So I had one of the first IBM PC's. Not counting my "Commodore". With the Internet, I choose Firefox as my browser and never had these issues before now with Firefox where you HAD to stay "leading, bleeding, edge", or when one upgrades, you are FORCED to use "create a new profile and lose everything". I suspect Mozilla is coping Microsoft with their arrogance where WE are here to serve THEM, not ourselves. With this Forums help, I was able to get my BOOKMARKS back but previously made the stupid mistake in trusting ADD-ONS like ONETAB with my BOOKMARKS, etc. ONETAB seem to be a VG concept but was poorly created and managed by its creators as no "assurances" of backup, protect against loss was considered.

Maybe a smarter group will come up with a more solid concept of "bookmarks", and/or "OneTab" needed for this decade and bring "PC's" into the 21th Century. I was also a "SUN UNIX SYS ADMIN". NOW THAT WAS SOME SOFTWARE and HARDWARE. As a Sun, Sys Admin at Motorola I used to tease my Microsoft counterparts, "One thing I can say you can not say "I don't remember the LAST TIME I re-booted MY System". We had a remote Sun Unix Server in a warehouse covered with junk for at least 5 years finally go down due to "release" changes, took us a week to find it to upgrade it and RE-BOOT it and it "kept on ticking'.

Thanks for your help.

It doesn't sound like the problem is solved:

With this Forums help, I was able to get my BOOKMARKS back but previously made the stupid mistake in trusting ADD-ONS like ONETAB with my BOOKMARKS, etc. ONETAB seem to be a VG concept but was poorly created and managed by its creators as no "assurances" of backup, protect against loss was considered.

Have you moved on from trying to recover data from the old OneTab file by this time?

Yes, [well maybe]. If I get the time with nothing to do, I may go back but from the possible fix I have read? It looks like it be quite a process.

I'm not sure of the best process. What I probably would try is to exchange some files. The problem is that it's difficult to identify the folders due to unique names per installation.