Hi, check to see if you have a 2nd Profile in here, date would be older that your looking for.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

If there is : you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem. Never work with original files.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

If not you got what it will give up to you.

Always :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Please let us know if this solved your issue or if need further assistance.

I don't have any old profiles. The profile is the same and fortunately it still contains data like my bookmarks etc. What it doesn't contain anymore is the session restore data, which I needed, because I would like to be able continue work from the point I stopped.

Does Firefox backup the files in sessionstore-backups in a proper way?

I understand that it may be a good idea to backup the profile, but I can't back it up just before Firefox or the system crashes, because I don't know when this will happen.

hfirefox said

Does Firefox backup the files in sessionstore-backups in a proper way?

No, but you might find a file that's "close enough" among the ones created when Firefox updates. At the risk of repeating what you've already found:

Could you start by making a backup of your existing session history files? Here's how:

Do not exit Firefox, or if you closed it, don't re-open it.

(1) To open your profile folder...

If Firefox is still running:

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" (or "Show in Finder") button.

If Firefox is closed:

Oops, sorry for pasting Windows stuff -- see Profiles - Where Firefox stores your bookmarks, passwords and other user data

Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated.

(2) Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

(3) What files did you find?

The kinds of files you may find among your sessionstore files are:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update
• various .js files from Firefox 55 or earlier

Could you take a look at what you have and the date/time of the various files to see whether you think any of them would have the missing tabs?

To preview the contents of a file, you can drag and drop it onto this page, then click Scrounge URLs: https://www.jeffersonscher.com/res/scrounger.html

That tool is on my site, so please let me know if it doesn't work for you.

By the way, are you certain you were using Tab Session Manager before the crash, and not the legacy extension Session Manager?

In the profile folder, look for a folder named sessions which would exist if you were using Session Manager.

i am using tab session manager yes