I’m having a heck of a time getting any package settings to apply. Everything in my profile works a treat though, so I’m doing something right. Here’s my config:
Could it be that in spite of restarts, there is an old firefox process still running - so it’s just creating new windows on an already-running firefox?
Also maybe check that you are running the firefox which home-manager has installed, not one in /run/current-system/sw/bin/firefox for example.