You should 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 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

Did you check "History -> Recently Closed Tabs/Windows" to see if you could reopen the window with your pinned tabs ?

Pinned tabs are specific for each window.

Backup the session files in the sessionstore-backups folder in the Firefox profile folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

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)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file with Firefox closed.

• make sure to backup the current sessionstore.jsonlz4

You can check a recent upgrade.jsonlz4-<build_id> file and check whether this file has the lost pinned tabs. You can look at this tool to inspect a compressed jsonlz4 sessionstore file.

• https://www.jeffersonscher.com/res/scrounger.html

Thank you for replying, cor-el. But my issue regards in-containers-tabs being restored out-of-any-container, and your answer doesn't mention containers at all.

Let's say I have these tabs in a window:

[Pinned][Container X] Site A [Pinned][Container Y] Site A [Non-pinned][Container Z] Site B [Non-pinned][Container W] Site B [Non-pinned][Container J] Site C [Non-pinned][Container K] Site D [Non-pinned][Container K] Site E [Non-pinned][Container Z] Site F

If I **accidentally** close that window (while having another window still open, of course), this is what Firefox restores when I tell it to restore that window:

[Pinned][Container X] Site A [Pinned][Container Y] Site A [Non-pinned][**no-container**] Site B [Non-pinned][**no-container**] Site B [Non-pinned][**no-container**] Site C [Non-pinned][**no-container**] Site D [Non-pinned][**no-container**] Site E [Non-pinned][**no-container**] Site F

This means, as I mentioned in my question, that **Firefox seems to lose container data for non-pinned tabs in (closed and) restored windows**. Is this a bug? Is is a temporary limitation yet to be solved by a upcoming feature? Is there a fix or workaround?

Thanks again.

EDIT: I just realized that all lost containers were in fact temporary containers. When trying to reproduce the issue, normal (non-temporary) containers are not forgotten/lost. Even if temporary containers were set to disappear not before 15min after the last tab was closed, apparently closing the window did something which the add-on, Firefox or them both couldn't handle. Anyway, it's a specific add-on issue only, not a general Firefox one, so I'll close this post.

I just realized that all lost containers were in fact temporary containers. When trying to reproduce the issue, normal (non-temporary) containers are not forgotten/lost. Even if temporary containers were set to disappear not before 15min after the last tab was closed, apparently closing the window did something which the add-on, Firefox or them both couldn't handle. Anyway, it's a specific add-on issue only, not a general Firefox one, so I'll close this post.