Newly created containers are sometimes wrongly numbered

[klaurence]

• Temporary Containers Version: v1.9.1
• Firefox Version: v83.0

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Actual behavior

These addon settings are set as follows:

• "Container Number" set to "Reuse available numbers"
• "Delete no longer needed Temporary Containers" as "After the last tab in it closes"

With these settings, newly created containers sometimes don't take the very first available number (in this case, any used number is made 'available' by closing every tab of the container using them).

Expected behavior

With the settings mentioned, newly created containers should take the very first available number.

Steps to reproduce

1. Set up "Container Number" and "Delete no longer needed Temporary Containers" settings.
2. Open new containers and close them in rapid succession. (Please note that sometimes it skips numbers not currently used, sometimes it doesn't, so I think maybe try opening new containers more than a few times)

[arthurl]

I've observed the same issue with the same setup as @KLaurence, and on top of that, numbers are sometimes reused. For example, "t1" and "t2" are repeated: