I have a personal preference for FF - mainly because I use it on the Mac's I have But in my ignorance I am unsure whether it is "safe" to install on Pi4 under Raspi.
Dunno why you would think it might not be safe on the Pi if it is safe on the Mac. Anyway, it's in the normal repositories -- try:
apt search firefox
I think the thing you want is
firefox-esr (but see below). Anything in the repo can be considered as "reasonably safe": Obviously any and all software is susceptible to bugs and security issues; Debian, which is the foundation of Raspbian/RpiOS, is very good at identifying and dealing with the latter.
WRT firefox, there's a bit of a weird history within Debian derived distros. Basically, there is/was a version of firefox called "iceweasel" that was used to circumvent some copyright issues. It's still in buster as "a transitional package which can be safely removed". I just mention this because you may see it referred to elsewhere particularly in older information.