Firefox uses sessionstore.js in the profile folder to store session data. Sessionstore.js is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to make is possible to restore the session at any time.

You can check in Windows Explorer if there is a previous version of the sessionstore file available.

• right-click: Properties -> Previous Versions (not sure about win 10 support)

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

You will normally find these files in the sessionstore-backups folder:

• previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
• recovery.js (latest version of the sessionstore written during runtime)
• recovery.bak (previous version of the sessionstore written during runtime)
• upgrade.js-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.js to replace the current file (make a backup copy of the current sessionstore.js).

See also:

• http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/

Hi, thanks so much for responding. I went to the Troubleshooting Infformation page and clicked the Show Folder button, and opened the sessionstore-backups folder. These are the only files present:

previous.js recovery.bak recovery.js upgrade.js20161129173726 upgrade.js20161208153507 upgrade.js20170125094131

What should I do next? I'm not sure how to find out if that session is still accessible somehow. Do I open the previous.js file with Firefox? Can you talk me through the steps please?

The three upgrade files are rather old as you can see by the install date of the version that got upgraded. You can check the file creation date the get the actual date of such a backup. The previous and recovery files are recent and probably from the last time you started Firefox and already have tabs with about:blank as name. Note that is sometimes can take a bit longer for Firefox to process sessionstore.js and have the actual names appear on the tab (I had times that it took several seconds). If you would interrupt this then you are likely to lose the content of these tabs.

If it happens again then do not close Firefox, but leave Firefox open and backup sessionstore.js and the files in the sessionstore-backups folder, especially previous.js that would have your previous session.

I understand what you say, but is there a way to open these files and view or copy the URLs manually somehow? Some way to locate them all? Even if it is a tedious, difficult and complicated job, I'd do whatever it takes, really.

Also, I've opened both recovery and previous files in Notepad but I don't know what I'm looking at and I'm just trying to find website names using the Find function.

A possible way to inspect a sessionstore.js file or a file in the sessionstore-backups folder is opening the file in a Firefox tab and evaluate this JavaScript code in the Web Console (Firefox/Tools > Web Developer). Paste the code in the command line of the Web Console and press the Enter key to evaluate the code.

• https://developer.mozilla.org/Tools/Web_Console

ss = JSON.parse(document.body.textContent);
sd='';
for (var i = 0; i < ss.windows[0].tabs.length; i++) {
 var ent = ss.windows[0].tabs[i].entries;
 var last = ent[ent.length-1];
 sd+="["+(i+1)+"]:"+"<a href="+last.url+">"+last.url+"</a><br />";
}
document.body.innerHTML=sd;

Thanks, but I can see that none of the tabs were remembered, which were open in the affected window at the time of the first crash. It's a huge dissapointment, even though they should all still be in my history, there's absolutely no way to restore them to the original browser context - there were far too many to even remember all of them individually.

I've downloaded a tool to help with that in the meantime, as it seems to be more reliable than Firefox itself when it comes to restoring sessions, but I'm looking for a new browser now. Firefox seems way too unstable/unreliable for me. Thanks for all your help, it's much appreciated, but it's the browser itself which is the problem.

One final question if I may - any ideas why it failed like this? (Edit: I'm referring to when the tabs failed to load properly in the first instance, after the previous crash.) It doesn't matter much if I'm going to switch browsers anyway, but in case I don't I'd like to know. Did I not upgrade to a new FF version soon enough to prevent this problem? Too many tabs? It had been managing (to reload the tabs) perfectly well until it failed on me. As I suspected, my mistake was expecting Firefox to be able to restore the tabs, knowing the risk existed that it wouldn't - but the same thing had happened once before and FF recovered just fine. I won't take it for granted next time, if there is a next time.

If it you see this happen the first time then you shouldn't close Firefox because that will definitely overwrite sessionstore.js and previous.js that might still have the tabs with a version that has them all as about:blank. You should leave Firefox open and make a backup of the files in sessionstore-backups and sessionstore.js before closing Firefox.

I've deleted both the sessionstore-backups folder files, and sessionstore.js after closing Firefox, just to start from scratch. Firefox is still pulling up a corrupted restore session file from somewhere. How can this be ? The problem is definitely reproducible.