The session has no window limit as long as you quit from the menu (Ctrl+Shift+Q). When you close some but not all windows, they go into the undo queue which is limited to three windows. You can increase this by going to about:config and changing browser.sessionstore.max_windows_undo.

There might be a session backup available with your lost windows if you are lucky.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

This might be too late by now, but were any additional windows listed on either:

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

Since it appears you updated Firefox recently, if you've had those windows open for a while, perhaps you can extract the list from a backup file. Here's how:

(1) Open your current profile folder

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

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

In the first table on the page, find the Profile Folder row and click the "Open Folder" button. This should launch Windows' File Explorer listing the contents of your current profile 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 -- this might be the most useful

Sometimes you can tell from the last modification time which one will have your missing tabs, but not always.

(4) Extract File Contents

These compressed files are a pain to view, so I created a tool on my website to list out their contents. If you want to try that, you can drag and drop it onto the large box on the following page, then click the "Scrounge URLs" button:

https://www.jeffersonscher.com/ffu/scrounger.html

If you don't get a list within 15 seconds, that probably means the script is caught in a loop. You may need to close the tab to avoid a tab crash and then try again in a new tab.

If you get a useful list, use the "Save List" button to archive it as a web page of clickable links for future reference, since you probably don't want to completely roll back your Firefox to that earlier date at this point.

Note: if you don't trust using my site, Firefox can decompress the file to its JSON-formatted contents, which you then could open in a tab to read. Here is a script to do the decompression: https://gist.github.com/jscher2000/4403507e33df0918289619edb83f8193