I feel a lot of the "complaints" about Firefox are misinformed opinions from a decade ago or more. I hear falsehoods such as "it's slower" or "it takes up more RAM" which are both patently false in modern times. That was all fixed up years ago.
(There's a steelmanned form of their argument - that Mozilla ought to be held to a higher standard than their commercial competitors - which you might think would speak in favour of LibreWolf and the like. But those who complain most forcefully about Firefox often turn out to be Chrome / Opera / Brave users)