Have you tried running without any extensions after having reset Firefox? The only time I've seen that where it wasn't a dying hard drive or a malware-infested/corporate IT-mismanaged system was when someone had some particularly poorly-optimized extensions installed.

I was curious & ran a couple of tests on this 2010 Mac Book Air. Normal launch time according to about:healthreport is 2.8 seconds and testing with something like `open -a Firefox https://www.google.com` when Firefox was not already running takes 4-5 seconds to finish rendering. When it is already running, it's under a second from start to finish.