Can you get the window if use the Command+backtick shortcut (⌘-`)?

On Windows, I would suggest the possibility that the window is off the screen (drawn on a second display or expanded desktop), or dysfunctionally small. However, I don't know whether these are potential scenarios that could occur on a Mac, or whether you have a keyboard shortcut to maximize the "current" window or otherwise bring it into view.

By the way, you may want to back up your session history files just in case you need to extract a list of the tabs in the missing window. Here's how:

(1) To open your 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 "Show in Finder" button. This should launch Finder listing the contents of your current profile folder. If instead of seeing numerous folders, Finder has selected a folder with a semi-random name, such as abc12359.default-release, 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

Optional File Contents Preview

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, in case no other approach is successful.

Thanks for your reply. The first idea just toggles between the 2 visible windows. it does not reveal the invisible third window which appears in the "window" menu. There are only 2 files in the sessionstore-backups folder. When I run them through your web tool I get three windows from 2021 which are just Firefox Welcome tabs.

Wow, that's strange about the session history files. Since Firefox restored three windows, it seems those files should be current and not two years old. Unless it actually did restore windows from two years ago rather than your recent windows...

Could you check for any additional profile folders? You'll need to back up a level in Finder to the Profiles folder and check there.

There also are add-ons to save your currently open tabs as a session snapshot. I haven't tested any of these myself in recent years, so my list might be a bit out of date, but you could take a look at these session managers and try whichever one sounds good to you:

• https://addons.mozilla.org/firefox/addon/tab-session-manager/
• https://addons.mozilla.org/firefox/addon/session-boss/
• https://addons.mozilla.org/firefox/addon/session-sync/
• https://addons.mozilla.org/firefox/addon/my-sessions/

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Do the files stored in the sessionstore.js get updated?

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

Hi thansk for your response. "Do the files stored in the sessionstore.js get updated?" No they apparently do not. As said, the only files there are from 2021 (created & modified).

OK same issue. The missing window finally appeared after 3 restarts.

On my latest restart, TWO windows are now invisible. i.e. they show int eh Windiow FF menu but not on screen. Help?

Do the missing windows have a diamond in front, indicating they are minimized to the Doc ?

No. I assume you mean in the FF Windows menu.

You can't reach them as well via "Command + tilde" ? To switch between Firefox windows, use: (Shift) "Command + ~" (Tilde) or possibly backtick (`)

Nope, nada.

I had the thought that you could use an add-on to move the tabs from the invisible window into one of the visible windows.

The following add-on merges another windows' tabs into the current window. I haven't tried it myself:

https://addons.mozilla.org/firefox/addon/window-merger/