This past weekend, my girlfriend decided to "help" me by closing most of my tabs and turning them into bookmarks, but I couldn't work with them in the order she put them in and wanted things back to way they were. I left the tabs as bookmarks, then went to my Library where I found a button that said "Restore" under the Import and Backup heading, and had it restore things to the way they were. Turns out it just got rid of the bookmarks, but didn't bring back my tabs. I really, really, need these back, and would rather not have to go crawling through my history for them. I have some of these tabs backed up from another pc on sync, but whenever I go to About: sync-tabs, it's completely blank. What do I do? Please help!
Step 1 would be to salvage your sessionstore files and make backup copies before exiting Firefox or making any other changes.
Open your active Firefox profile folder (personal settings folder) using
Help > Troubleshooting Information > "Show Folder" button
Copy all files that start with sessionstore to a safe location such as your documents folder.
If you have not exited Firefox since this happened, try the History menu to re-open closed windows ("Previously closed windows") and within each window, to re-open closed tabs ("Previously closed tabs"). Can you get them back??
Unfortunately, by default, Firefox only stores 10 "Previously closed tabs" per window and 3 "Previously closed windows." You can find these lists on the History menu. This might be enough for most occasions, but you can increase the number stored.
(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the search box above the list, type or paste sess and pause while the list is filtered
(3) Double-click the browser.sessionstore.max_windows_undo preference and enter your preferred number of windows (I have 10).
(4) Double-click the browser.sessionstore.max_tabs_undo preference and enter your preferred number of tabs (I have 20).
If you exited Firefox after the tabs were closed, does the sessionstore.bak file have a promising date/time stamp around that time? If you have exited and restarted Firefox multiple times, it's probably no longer possible to recover the old session data. To keep numerous sessions, you need an extension like Session Manager or Tab Mix Plus.
I changed the max windows and max tabs, but it's still not showing anything differently from what was there before. The session restore data is promising, but it looks like it was from after the tabs were closed, but before the bookmarks were gone, so what do I do from here?
Does the previous version of a JSON backup have those lost bookmarked tabs? Restoring a JSON backup replaces all current bookmarks, so best is always to backup the current bookmarks before restoring such a bacup.
Open tabs are stored in the sessionstore.js file in the Firefox profile folder.
Can you bring back a previous version of the sessionstore.js file?
You can try to restore a previous version of the sessionstore.js file of a date that has those tabs.
The bookmarks backups are from before and after this happened, so they don't have the bookmarked tabs. I looked into the sessionstore.bak file, it seems to be from about the time when the tabs were taken out, but when I've tried to restore my session it's always shown up without the tabs, and I don't know where to get any other version of it. I use Windows Vista, and can't seem to get the previous versions thing to work. Would system restore be able to help me here? Any other advice?
Those previous versions come from files in a System Restore point AFAIK, so if there aren't any promising previous versions then System Restore probably won't work either and SR can have other ill effects that can break your Firefox installation, so best is always to be cautious with using SR.
There are some ways to look for lost URLs in a sessionstore.js/bak file. These threads have suggestions on how to "mine" the file to extract the URLs: