Hi stephitzj, sorry to hear about this problem.

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

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

If they are missing or grayed, check either:

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

Can you re-open any closed windows that might have the missing tabs?

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:

Do not exit Firefox, or if you closed it, don't re-open it.

(1) To open your profile folder...

If Firefox is still running:

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" (or "Show in Finder") button.

If Firefox is closed:

Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated.

(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
• some old .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/ffu/scrounger.html

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

Hi jscher2000, Many thanks for your help with this!

There is no Restore Previous Session under either my 3-bar menu or under History, and the Recently Closed Windows simply gives me the list of pages that I've viewed since this morning trying to figure out what has gone wrong!

However, I've had a look at my Firefox profile folder and its numerous semi-randomly named folders. I also found the backups, which included that from 4th August. I have run that through your SCroung URL tool and, although it translates it (very many thanks for that!) it just keeps scrolling when I select Scrounge URLs. In the end (because time is pressing and I have work to do!) I have copied the translated text into a Word document and am doing a search and replace of "uri": - which prefixes all of the urls.

So, again, many thanks for your help: my main concern is that this is going to happen again so, for the time being, I am keeping the Word document of all my urls as a ready-to-hand back up!

Come on Mozilla - please don't shake my confidence!

Hi stephitzj, Firefox shouldn't lose sessions. I think it happened to me once in recent years; fortunately I already knew what folder to immediately go and back up before doing anything else. Now you do, too. ;-)

If you might want extra "insurance," there are several session management extensions you could consider. 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.

jscher2000, Many thanks once again. My fingers are crossed that this doesn't happen again!