Private tabs killed if created shortly after browser startup

[c-d-a]

What happened?

If a private tab is opened (Ctrl+Alt+T) within a few seconds of launching a browser window, Waterfox will remove it. This can happen multiple times in a row, and happens even if there are already other Waterfox windows open.

Additionally, if the killed tab was the last one remaining, it will be replaced with a non-private homepage tab, even if new tabs are set to open a blank page (not home) in the settings.

This is a regression that happened alongside the ESR128->ESR140 switch.
6.5.11 worked fine, 6.6.0 introduced the bug.

Edit: possibly related to #4114 ?

Reproducible?

• I have checked that this issue cannot be reproduced on Mozilla Firefox.

Version

6.6.12

What platform are you seeing the problem on?

Windows