Never saw it before, but going by its description now, it’s the “GNU version of the Firefox browser”, so I would presume you can import everything you could between two Firefox installations, like whole profiles and favorites back up file.

Icecat is a really awkward name. :)

You should be able to just copy the contents of the Firefox profile folder over the contents of the Icecat profile folder. Make sure neither is running at the time.

Don’t do it, use something like !librewolf@lemmy.ml instead

It actually offers meaningful privacy protections at no big cost in terms of usability.