There used to be an option called browser.privateWindowSeparation.enabled, which ever since Firefox 106 back in October 2022 resulted in separate taskbar icons for normal and private Firefox windows. In version 127, this option has gone away, removed in this code change. Users complained on Mozilla’s forums and on Reddit at the time, but it was at least possible to recombine the icons with the option in about:config – but no longer.
As you might imagine, people are not happy, although according to the official response in this complaint, it looks like the change will be reverted in Firefox 128…
Yeah, about:config is one of the best things about Firefox. It allows a standard user not to stumble into settings that would lead to frustration or needing help. But it also lets power users use Firefox the way they want to. I’m always annoyed when a setting is removed from there.