Bookmarks are not same a sessions that would be stored in the History. But if your system lost the sessions or you closed and opened and closed again without restoring the sessions they will be gone. Look at you History to find the closed sessions if they are there. Otherwise if your system is set to not remember history or sessions then they will be gone when you close the Browser.

The Bookmarks are still still there. Ok, now I know.

Thank you for your answer.

Do you have Firefox set to restore previous session windows and tabs automatically, or is it more of a situation of Firefox behaving as though it is recovering from a crash at every startup?

Could you check whether either of these are available (not grayed out):

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

If they are grayed, 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... If you tend to keep the same tabs open for a long period of time, you might find them in an older session history file.

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

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

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

In the first table on the page, click the "Show in Finder" button. If Finder has highlighted a folder with a semi-random name like a1b2c3d4.default, click into that folder.

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location, or you can copy/paste this folder to your desktop.

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 any of them are likely to 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.