Might depend on when you first launched Firefox, so it tries to "grandfather" some settings across the versions?
That `ctrl + .` now opens the emoji picker on Linux seems to very much be intended, judging by this release post: https://www.firefox.com/en-US/firefox/150.0/releasenotes/
> Added support for the GTK emoji picker on Linux, allowing users to insert emoji using the system shortcut (typically Ctrl+.).