In the Option settings, Startup section should have the check box for Restore Previous Session. Or see if your History still keep such tabs. Not sure if this is what you are looking

Hi stub, if Restore Previous Session is missing from the menu, then either:

• Firefox did not find the previous session history files
• Firefox could not use the previous session history files
• Firefox already restored the previous session

In the case of #3, check either:

• "Library" toolbar button > History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)
• (menu bar) History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)

If that doesn't help, could you start by making a backup of your existing session history files? Here's a rough guide from a Windows person; your system will vary.

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"/"Open Directory"/"Show in Finder" button.

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

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.

Note that if you use the Firefox version from the Ubuntu repositories that this version might behave differently than the regular release from the Mozilla server.

You might want to check for a upgrade.jsonlz4 file in the sessionstore-backups folder as posted above.

• upgrade.jsonlz4-<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.jsonlz4 to replace the current file.

• make sure to backup the current sessionstore.jsonlz4 file