I have been using Firefox, and only Firefox, on the desktop since before Firefox 1.5 (released in 2005) and Firefox has had plenty of performance issues over the years and still have some. The performance issues still reaming after the Quantum release are mostly caused by shitty websites who only optimize their crappy code just enough for it to work in Chrome. A couple of culprits which slow down my browser significantly are Trello and Twitch. Another issue is that 3D acceleration does not work well on Linux so it is disabled by default and often cannot be enabled without compromising the stability.
Before Quantum on the other hand I had to regularly restart Firefox since it seemed to slow down over time and hog all of my CPU no matter if I had many or few tabs and no matter if I closed tabs so I am not surprised many got burned by Firefox and how it could not handle today's bloated websites until Quantum. The UI also got jankier and jankier the more CPU Firefox started using.
Chrome probably has had its own performance issues but I cannot comment on them since I have not used it much other than on mobile.
Before Quantum on the other hand I had to regularly restart Firefox since it seemed to slow down over time and hog all of my CPU no matter if I had many or few tabs and no matter if I closed tabs so I am not surprised many got burned by Firefox and how it could not handle today's bloated websites until Quantum. The UI also got jankier and jankier the more CPU Firefox started using.
Chrome probably has had its own performance issues but I cannot comment on them since I have not used it much other than on mobile.