Firefox creates sessionstore.js at shutdown. So you need to exit out of Firefox completely, then rename sessionstore.js to something like sessionstore.old, copy in your other .js file and rename it to sessionstore.js. When Firefox starts up, it should read the sessionstore.js file and then delete it. (Or perhaps it moves the file to the sessionstore-backups folder and renames it to previous.js, I haven't watched exactly what happens to it.)

I've tried exiting out of Firefox (and checked the Task Manager to see if it was still running in the background and closed that as well) but there will still be no sessionstore.js in the profile folder even when Firefox is closed.

And no matter what tabs I open before closing Firefox now, it will just open up my homepage the next time I open the browser. I haven't changed it in the settings to do that; it's supposed to open my previously opened tabs, but it's just not doing that anymore at all, so there's another issue, or an extension of my existing one.

I think if you have to "kill" firefox.exe using the Task Manager, then it won't be able to create sessionstore.js at shutdown and is more likely to resume using recovery.js (in the sessionstore-backups folder), but this is not something I have studied in detail.

Is Firefox creating and updating recovery.js during your session?

Do you want to try renaming recovery.js and substituting in your backup as the new recovery.js?

I think you just hit the nail on the head with the point you made about having to kill firefox.exe using the Task Manager. After I exited Firefox earlier, the Task Manager did say it was closed under "Apps" but still running under "Background processes," where I had to manually close it, and there was no sessionstore.js in the profile folder before I did that.

Now I checked the Task Manager again and since I closed the background process several sessions ago, it wasn't running anymore and a sessionstore.js had finally appeared in the profile folder. I guess manually closing the background process was what allowed Firefox to create the file and made all the difference then?

So now I was able to replace sessionstore.js with my backup and it actually worked. Problem solved!

And on the off chance this might be useful to anyone else - replacing recovery.js with my own backup didn't work. I tried it before and it would just delete my backup and replace it with a new, clean recovery.js with no tabs in but my homepage upon reopening Firefox.

Thanks a lot for your help, jscher, I'm really glad I could get my tabs working again.