Unfortunately, session history is only for the immediately previous session, so if they opened and closed Firefox without restoring the last session, it's probably not going to be possible to retrieve it in full.

Have you already tried either:

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

If they are grayed, then 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...

Firefox creates extra session history files when it updates, so that might be the best we could do. Please grab a backup of your existing session history files as follows:

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.

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

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 modified 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, I'm assuming you didn't find a new "Old Firefox Data" folder on your desktop. That would be created if Firefox's Refresh feature ran. Unless you asked them to troubleshoot Firefox specifically, they probably wouldn't have run it. However, if you do see that folder, click inside and look for the sessionstore-backups folder there, as that also could be useful.

Also, just in case, some users say "tabs" when they are referring to buttons on the Bookmarks Toolbar.

You can use one of these methods to show the toolbar list and redisplay that bar:

• right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the "+" button)
• (menu bar) View > Toolbars
  On Windows, tap the Alt key to activate the menu bar temporarily.
• in Customize mode > Show/Hide Toolbars (see: Customize Firefox controls, buttons and toolbars)

To turn on the Bookmarks Toolbar, Menu Bar, or other bars, click it on the list.