You can pull the network cable and sign out of your Firefox Account -- declining any option to delete your data -- if the problem is Sync.

Regarding the settings loss:

Which part is lost? The "container" of an open tab is stored in your local session history. Any list of sites associated with specific containers is add-on data stored by the Multi-Account Containers extension. What was syncing before?

I'm not sure if sync is the cause of my containers issue, but the problem is spreading to my other computers with sync. I had my system setup with ten containers, and those are all gone now. So I'm left with the original 4. I believe all my devices were in sync, and the problem started on my linux laptop, and then moved to my windows laptop. I'm hoping I can salvage my container setup, by disconnecting my windows desktop from the sync chain. Is he container setup in a local file, that I can backup?

Each profile folder has a containers.json file which should have your container name, color, and icon choices, along with a userContextId number that is stored in your session history along with the tab title and URL. Please back up that file. To read its contents, you can drop it into an open tab in Firefox and a structured view should display.

What I don't know is whether you can safely drop that file into a different profile, because the userContextId probably depends on the order the containers were created. It would be safer to look for backups of the file made on the same system.

That sounds rather complex :) In what folder do I find that file?

You can use the button on the "Help -> More Troubleshooting Information" (about:support) 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

I found the containers.json file. On my windows laptop it has only the 4 original containers. But to my surprise on my windows desktop, all my containers are still there :) Even after a Firefox restart and an extra sync. So how do I get this correct containers setup, to sync to my windows laptop again? Do I have to copy over the containers.json file, or should I leave and rejoin firefox sync?

I found the containers.json file. On my windows laptop it has only the 4 original containers. But to my surprise on my windows desktop, all my containers are still there :) Even after a Firefox restart and an extra sync. So how do I get this correct containers setup, to sync to my windows laptop again? Do I have to copy over the containers.json file, or should I leave and rejoin firefox sync?

So I think you have caught up to my earlier ruminations. You can copy the file over, but if it doesn't match the session history -- in other words, if the userContextId of restored tabs was different on the laptop -- then tabs could end up in the wrong containers. Maybe that isn't as much of a PITA as re-creating them from scratch? Or maybe Firefox flushed the old assignments by now anyway?

I just checked my laptops (linux and windows) Both are fine now, And I didn't change anything. I wonder if this problem has something to do, because I got both librewolf and firefox installed on linux. But for now, my problems are solved :) Thanks for the help.

On a side note; where is the data stored, that assigns websites to certain containers?

I don't know about the other browsers. Presumably they aren't sharing data with Firefox?

On the side note:

By default, Firefox matches tabs with containers, and not websites. Matching websites with containers is a feature of an add-on like the Multi-Account Containers extension. Add-on storage (under [profile.folder]\storage\default) uses in a locally uniquely named moz-extension folder in a database in a compressed form.

Some add-ons use Sync storage, and others use Local storage (Local doesn't sync). The reason they don't all use Sync storage, besides possibly not being relevant, is that Sync storage can fill up quickly due to a low cap, and if the extension can store a lot of data, the user will stop being able to save new data once that cap is hit.