Your comment reminds me of how people were complaining for a long time that FireFox had become so slow, while I was browsing with multiple dozens of open tabs on my 8GB system all along. I do not want to say that this is not the case for you, but I honestly cannot reproduce it on three different laptops.
To be fair, Mozilla does deserve 99.999% of the hate it gets, but that doesn't justify making dishonest claims about Firefox just to stick it to Mozilla.
(I should probably mention, another long-running complaint I have with Firefox is the community's habit of gaslighting anybody who talks about Firefox's problems with the usual "I never experience that so your experiences are invalid" So please spare me this time.)
EDIT: by "modern" I mean I'm on an I5 with 8GB RAM lol. I do have a Lenovo Legion 7 with a 3080 but use it only for gaming. This I5/8GB combo is my daily driver and works great with Firefox, loads instantly and I can have a dozen or more tabs open no problem.