If a startup tab is closed before fully loading, it reopens in a temporary container

[e-t-l]

• Temporary Containers Version: 1.9.2
• Firefox Version: 90.0.2

Actual behavior

I have startup behavior set to open several URLs. Upon launching Firefox, if any of these tabs is closed before fully loading, a second later it will reopen in a temporary container. None of these URLs/domains are set to be isolated in Temporary Containers, and automatic mode is disabled. (note: all TC settings are at their default values, with the exception of a few isolated domains unrelated to this issue).

Expected behavior

When a tab is closed manually, it stays closed.

Steps to reproduce

Not reproducible in a new profile :/

Notes

• Disabling the Multi-Account Containers addon did not help, although disabling TC did prevent this issue from occurring (I found this informative. If the root issue wasn't with TC, then maybe the tabs would've reopened in normal containers. The fact that this issue only occurs when TC is activated makes me think it's something caused by this extension.)
• I have some non-default about:config prefs, but these are managed by a user.js file. I tried creating a new profile, installing MAC and TC, and copying over the user.js file, but the issue was still not reproduced. This indicated the issue is not with a pref.
• I also have some custom chrome styling with userChrome.css, but I can't fathom how that could be relevant. Still, adding it in case anyone knows something I don't.
• I tried resetting (as well as uninstall/reinstalling) the TC addon with no effect.
• ~My best guess is that TC is interacting with some other extension, but I don't know which. I'll update this issue as I go through my extensions and disable them one-by-one to test for reproducibility.~
• UPDATE: I went through my addons, disabling them one-by-one and retesting. Unfortunately the issue persisted throughout, which means it's not caused by another extension.
• I guess it could be some pref that got switched at some point in about:config itself. I'm not sure the best way to test for this. (I'd rather not have to refresh my whole FF profile, as this is just one bug and I do have a lot of customizations noted above that I like. That being said, it sure would be nice to know how to fix this one particular bug)

ABOUT:SUPPORT OUTPUT FILE ATTACHED. support.txt

[e-t-l]

This seems to be a bug related to the setting Advanced > General > Isolation > Multi-Account Containers. When this setting is disabled, the issue resolves. May be related to #521.

[stoically]

Sorry you're encountering that bug, but

Not reproducible in a new profile :/

doesn't make it actionable. It's way too much effort to try to maybe find the combination of settings that causes it for your. Best advice is starting a profile from scratch until you hit the issue again, then not toggle the setting it causes and report it here, making the issue actionable.