I've found some stuff to be a bit slower on new FF since I made the switch when Quantum came out, but I chalk that up to heavy use of some Google webapps in which the chrome optimizations might have the opposite effect on a different JavaScript engine, and heavy use of containers, which are awesome.[1]
1: At first I didn't know how to get sites to default to a container, so kept having to close and reopen tabs to get the right contianer, but once I figured that out, it's smooth.
I too experience surprisingly bad performance with google webapps in Firefox.
I've had FF become extremely slow on two machines while it worked fine everywhere else. After a few months, back to normal. Profile refreshes etc didn't seem to make any difference and I still have no idea why it happens.
I've been om FF the while time and I'm impatient.
In the other hand I've no reason not to believe it when so many people report performance issues.
My best guess is a combination of different browsing habits and/or different hardware/os.
(My browsing habits are mostly hn, local news and pages I work on and I've had decent hardware running various versions of Linux and Windows.)