I'm not sure exactly what is going on with your Firefox. (If this is an emergency and you need to recover older session history files, please see the next reply.)

First, there seems to be a difference on Mac that if you close all the windows, Firefox continues running until quit the program, but in that scenario the next starting state is all windows closed instead of the last-closed window being re-opened. I don't know whether there is a general workaround for that other than making sure your main window is open when you choose Firefox > Quit.

Second, you can check whether Firefox is still recording session history by looking at the timestamp on the recovery.jsonlz4 file. You also can back up that file from time to time. Here's where it lives:

[profile folder]\sessionstore-backups

To open your profile folder, either

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

In the first table on the page, Profile Folder row, click the "Show in Finder" button. If Finder shows a folder selected with a name like ab4321cd.default-release then double-click into that folder.

Third, as a supplement to the built-in session restore feature, you could consider using a session management extension to store open tabs. These are the ones I'm aware of that you might investigate/try out:

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.

If these --

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

-- are missing or grayed, check either:

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

If that doesn't help...

Firefox creates numerous session history files, but because session history is only for the immediately previous session, it is unfortunately too easy to lose it. Could you start by making a backup of your existing session history files? Here's how:

(1) To open your profile folder...

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

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

In the first table on the page, Profile Folder row, click the "Show in Finder" button. If Finder shows a folder selected with a name like ab4321cd.default-release then double-click into that 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

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/ffu/scrounger.html

That tool is on my site, so please let me know if it doesn't work for you.

jscher2000 said

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

Hello jscher2000, Thanks for the tip that led me to a quite interesting workaround!

As I said, I always keep all the tabs in just one window. But, reading your detailed response, I decided to give a try to this one (as obviously, Restore session option was greyed out.

So, I tried the History > Recently Closed Windows - and Firefox suddenly opened 3 windows in a shot. The front one was empty, the last one too, but that second one included all my pinned tabs from the last run!

So thanks a lot! I don't think dev team would ever read this thread, so marking this as resolved, at least this works and might help someone else.