You can't just copy the old data back that's not how it work as the new install is different from the old backup copy you copied. This will cause Firefox to corrupt or delete those data and create new data. What should've been done was using Firefox FAQ on how to backup and restore the data back is what should've been done first with matching board or have a computer shop properly backup the data using Firefox FAQ so the data can be restored back correctly. I doubt that Win11 install would use the old data as that was formatted and created for a different Win OS and will not load properly.

Well as I said, I installed Win11 and FireFox from scratch because my old drive is no longer bootable on newer hardware. So whether I should have thought ahead and done a FireFox backup or not is irrelevant. I did not have the opportunity to do so as the old motherboard died and the new one wouldn't let me boot from the old drive.

Second, not only have I moved both profile and tabs before - as I said in my question - others have, too. And there are instructions on a support.mozilla.org page to do it, so I'm pretty sure it's just fine. In fact, as I said in my posting, I succeeded in moving my entire FireFox profile. No problem. My main password returned, my old settings are here, my history is here, bookmarks, all kinds of things which indicated that I am operating FireFox under the old profile. The entire profile is here and I'm using it to write this. Even my old cookies are here. And I'll bet that all of my old tabs are here, too. But I can't restore them. The button is not there. It isn't just grayed out, it's not there. Has it moved? Has it been deprecated? Has it been renamed? I don't know. That's what I'm asking about.

I have actually reloaded FireFox tabs before without problems. The capability is (or was) built into FireFox. You copy the contents of the sessionstore-backups into the same location in the current profile and then go to History > Restore Previous Session to recover them. The Restore Previous Session button can be found in other ways like starting from the menu or from the 3bars or from the History Library, etc. But I can't find it anywhere now and THIS is what my question is about. It does not appear that I have the ability to Restore Previous Session and I want to know why and how to get it back. Or has it moved to some hidden place in a recent FireFox release and the various online helps that can be found by web searches are now out of date?

Brian, in my history menu (the one that opens after pressing alt+S) I have "Restore Previous Session" as the third item. Is that not there for you?

Or have you tried pressing ctrl+shift+T to see if that restores the session?

If that doesn't work then you could try manually restoring a session from a backup of your old profile, if you have one. See this article for the steps: https://support.mozilla.org/en-US/kb/how-restore-browsing-session-backup

Correct. That is what I said. "Restore Previous Session" is not there. There are usually multiple ways to get to that button, but none of them lead me to it. It is the key that I need. Is there some setting that makes it disappear?

I did try ctrl+shift+T at one point and it did not help. But honestly, so much has happened that I'm not sure exactly what point in my troubles I tried it. I'll try and get myself setup for a new attempt and try it again.

I do not have a backup of my old profile. I have the entire old profile on the old disk drive which is currently available as an additional drive in my new system. But I don't think that will help. The old profile is not an issue. Getting the backed up tabs into the new installation is not an issue. Restoring the backed up tabs is the issue.

Step 1: Locate the Old Profile Data

Make sure you have copied the entire old profile folder correctly. The profile folder typically contains several important files, including session data. The profile folder is usually located in:

   C:\Users\<YourUsername>\AppData\Roaming\Mozilla\Firefox\Profiles\

Step 2: Check for Session Files

In your old profile folder, look for the following files:

   recovery.jsonlz4
   previous.jsonlz4
   upgrade.jsonlz4
   sessionstore.jsonlz4

These files contain session data, including your open tabs. Step 3: Copy Session Files to New Profile

   Close Firefox completely.
   Navigate to your new profile folder (the same location as above).
   Copy the session files (recovery.jsonlz4 and previous.jsonlz4) from your old profile folder to the new profile folder, replacing any existing session files.

Step 4: Restart Firefox

   Open Firefox again.
   If the session files were copied correctly, Firefox should restore your previous session, including your old tabs.

Step 5: Use the Session Restore Feature

If the above steps do not work, you can try the following:

   Open Firefox and type about:sessionrestore in the address bar and press Enter.
   This should bring up the session restore page, where you can see if your old session is available to restore.

Step 6: Check the History

If you still cannot find your old tabs, you can check your browsing history:

   Click on the menu button (three horizontal lines) in the top right corner.
   Select Library > History > Show All History.
   You can search for specific sites or scroll through your history to find the tabs you had open.

Step 7: Use a Backup (if available)

If you have a backup of your old profile or the Firefox data, you can restore from that backup. Additional Tips

   If you have any extensions that manage sessions or tabs, ensure they are installed and configured correctly in your new Firefox installation.
   If you have trouble with the session files, you can also try using a session manager extension from the Firefox Add-ons site to help manage and restore sessions in the future.