I've asked a similar question years ago regarding previous instance opening in a different virtual desktop with no joy. I simply close the window and launch a new Firefox instance on my working desktop. If I want to go back to a previous site, History presents itself.

Here's the answer to the question I was able to fixed it after reading a few things.

Make sure that when you do this your firefox is closed.

type this to a blank tab about:support

find Profile Folder, it will open a windows explorer leading you to something like this: C:\Users\(your pc name)\AppData\Roaming\Mozilla\Firefox\Profiles\eklf8yiy.default-release(this may vary)

look for sessionstore-backups, then rename it as sessionstore-backupsOLD

then open the sessionstore-backupsOLD folder

then there is a file something like upgrade.jsonlz4-20250806102122 (the numbers at the end may vary) if you have multiple files make sure to test out each file.

now go back one folder back [C:\Users\(your pc name)\AppData\Roaming\Mozilla\Firefox\Profiles]

From the profiles folder look for sessionstore.jsonlz4 rename it as sessionstoreOLD.jsonlz4

then copy the upgrade.jsonlz4-20250806102122 from the sessionstore-backupsOLD to [C:\Users\(your pc name)\AppData\Roaming\Mozilla\Firefox\Profiles] and rename it to sessionstore.jsonlz4

after that you can test out the other upgrade.jsonlz4-xxxx files that you may have from the renamed folder sessionstore-backupsOLD

I closed this as I resolved my own issue. Just refer to the reply I made