I respect your individual experience, Firefox may be slow for you on Linux. It's not for me.
That being said, I don't believe this kind of performance is the deciding factor in choosing your web browser any more. All of them are doing at least "Okay" in browsing the web. Other things seem to be more important, at least to me. For example, Firefox (and Mozilla) are not without faults, but they surely are one of the few obstacles remaining to prevent a Chromium monoculture and a fall-back into IE4 times.
I've put up Firefox as my default for a long time. But the gap in performance is so big, now, that I find it painfully slow to use. Recommend you try the latest brave, just so you can see for yourself what performance is possible, and how much nicer it feels to have a snappy browser.
All Mozilla need do is fix the longstanding performance issues and they'll win me back.
That being said, I don't believe this kind of performance is the deciding factor in choosing your web browser any more. All of them are doing at least "Okay" in browsing the web. Other things seem to be more important, at least to me. For example, Firefox (and Mozilla) are not without faults, but they surely are one of the few obstacles remaining to prevent a Chromium monoculture and a fall-back into IE4 times.