Sorry to hear about this problem. Could you check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > History > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, check either:

• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

If there's no items to restore the session manually, or they don't restore, here are three potential issues to investigate:

(1) Firefox isn't save session history as you browse (less common) (2) Firefox is deleting your history when it closes (3) An external program is deleting your session history between sessions

But first...

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 > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, find the Profile Folder row and click the "Open Folder" button. This should launch Windows' File Explorer listing the contents of your current profile folder.

If Firefox is closed:

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

Sometimes you can tell from the last modification time which one will have your missing tabs, but not always.

Optional File Contents Preview

These compressed files are a pain to view, so I created a tool on my website to list out their contents. If you want to try that, you can drag and drop it onto the large box on the following page, then click the "Scrounge URLs" button:

https://www.jeffersonscher.com/ffu/scrounger.html

If you don't get a list within 15 seconds, that probably means the script is caught in a loop. You may need to close the tab to avoid a tab crash and then try again in a new tab.

If you get a useful list, use the "Save List" button to archive it as a web page of clickable links for future reference, in case no other approach is successful.

If there's no items to restore the session manually, or they don't restore, here are three potential issues to investigate: (1) Firefox isn't save session history as you browse (less common) (2) Firefox is deleting your history when it closes (3) An external program is deleting your session history between sessions

For #1, as you browse, are you able to restore closed tabs using either Ctrl+Shift+T or the History menu's Recently Closed Tabs list? Similarly, are you able to restore closed windows using either Ctrl+Shift+N or the History menu's Recently Closed Windows list?

For #2, could you make sure that you are not clearing history at shutdown. On the Settings page, Privacy & Security panel, History section, there's a general selector:

• Firefox will: Remember history => should be okay
• Firefox will: Never remember history => prevents restoring your session
• Firefox will: Use custom settings for history => need to check the details

With custom settings:

[_] Always use private browsing mode => must be un-checked [X] Remember browsing and download history => must be checked [?] Clear history when Firefox closes => use the Settings button to check what is being cleared and make sure that "Browsing & download history" is not set to be cleared

For #3, programs like CCleaner and Advanced SystemCare may clean browser data at system startup or between sessions. You should be able to set them not to touch Firefox folders.

(2) Firefox is deleting your history when it closes

It is. I have had this option on for months, without any issues. Why did it work fine for all that time, then suddenly change its mind?

I just want to save open tabs, I don't want to save browsing history besides that. Are you saying that is/should be impossible?

Session history is considered part of browsing history because it is full of URLs you've visited. I wonder whether there was a bug that caused Firefox to fail to clean out those separate files, and that bug was fixed recently?

If you do not need to save the back/forward history of each tab, then there are add-ons which can save and restore tabs even if you clear your history at shutdown. These are the ones I'm aware of that you might investigate/try out and there might be new ones since I did my survey:

Use database storage:

• https://addons.mozilla.org/firefox/addon/tab-session-manager/
• https://addons.mozilla.org/firefox/addon/session-boss/

Use bookmark storage:

• https://addons.mozilla.org/firefox/addon/session-sync/
• https://addons.mozilla.org/firefox/addon/my-sessions/

Bookmark storage is more robust in that Firefox backs it up regularly. However, having the extra data show up when you are searching your bookmarks may be annoying.