Hi ragnarok972, your screenshot indicates that the Firefox Multi-Account Containers extension is enabled. You can confirm that on the Add-ons page, Extensions section (see attached screenshot).

You should find a button for the extension on the main toolbar:

Is there a problem with the toolbar button?

but still "firefox multi account containers plugin needs container tabs"

ragnarok972 said

but still "firefox multi account containers plugin needs container tabs"

Yes. That message is the explanation why the box that is below the message cannot be changed. This is normal when you have the extension installed and enabled.

thanks a bunch!