see https://support.mozilla.com/en-US/questions/841322

Use File > Exit to close Firefox

To recover as much as you can

1. Open History menu (Alt to show menus if don;t show them)
2. Restore Previous Session
3. Recently Closed Windows > individual or Restore all windows

Restore previous session, recently closed tabs and recently closed windows are all greyed out. No love there. Sorry.

If you want Firefox to open that session then try to select :

• Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time"

This was how Firefox was set to begin with. As far as I know, that setting has never changed. Firefox just doesn't follow this command.

I had the same problem. It seems Firefox cannot handle the sessionstore.js if it is very big. My was nearly 170 MB big! I didn't tried to understand the file format but it seems the important part is before the string "_closedTabs". I removed everything from there to the end and added the corresponding end of a fresh sessionstore.js. Then I tried to start Firefox again, it computed some time (maybe to restore the essential part of the end) and then I had my tabs back :-) Now the size of my sessionstore.js is 785 KB.

If you have also such a big file, you probably don't want to use an editor to remove the big part. I did it with sed in the command line of a Linux:

sed 's/_closedTabs.*//g' sessionstore.bak > sessionstore.restore

I'm not an expert in command line tools and I bet there is a better way, but it finished fast enough.

My sessionstore.js was just 602 KB, but I had lost my open tabs anyway. I have found a similar solution as nexxTM.

After discovering the loss, I immediately made a copy of sessionstore.js. I opened it in a file editor. Unreadable. I found out it is in JSON format, so I downloaded a JSON plugin for my text editor (in my case, JSON Viewer for Notepad++) and formatted it. I became accustomed with the file format to some degree. I realized all my lost tabs were in a section _closedWindows. I cut its content and pasted it to the section windows, removing the old content. Then I undid the formatting, because Firefox cannot handle it. I needed to remove all new line and tab characters. In my case, this was a safe operation as the original file did not contain these characters. Then I moved the repaired file to the profile directory and started Firefox. My tabs were back.