During crashes the chance to restore sessions is zero as nothing was saved during the crashes. Only if it was closes can one restore the sessions assuming you restored them before closing the browser if you did that there us no sessions as they would be in the history listing only then.

Sorry to hear about this problem. Are previous session tabs the only data that is failing to show up when you start Firefox? Your browsing history is still intact on the History menu?

When Restore Previous Session is grayed, either:

(A) Firefox already restored the session, so try either:

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

Note: Each window also has its own Recently Closed Tabs list.

(B) Firefox wasn't able to recover from the latest session history file or any of its (few) backups

That's very unfortunate. Usually a system crash isn't that destructive. However, this could be caused by using features of Firefox that clear browsing history (open tab URLs are considered part of browsing history when it comes to clearing data).

If you want to search out possible Firefox data files:

(1) To open your profile folder...

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/Return

In the first table on the page, find the Profile Folder row and click the "Open Folder"/"Show in Finder" button. This should launch MacOS Finder listing the contents of your current profile folder.

(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 you use the MacOS feature "Time Machine" you might have a backup of that sessionstore-backups folder in there. As a Windows person, I'm not personally familiar with how to check for that.

During crashes the chance to restore sessions is zero as nothing was saved during the crashes. Only if it was closes can one restore the sessions assuming you restored them before closing the browser if you did that there us no sessions as they would be in the history listing only then.

You can restore previous session after a crash https://support.mozilla.org/en-US/kb/restore-previous-session#w_after-a-crash