I have Firefox 64.0.2 running on my home computer. They better resolve the occasional slowdown issues with the newly-release Firefox 65.0.
65 now? I’m still running 57- didn’t realize it had gone that high lol
I only have it as portable, since otherwise it takes over Firefox ESR, disabling most all the extensions, without which Firefox becomes more like Chrome, with vastly reduced functionality.
But slow downs are mostly likely due your provider or other issues. I use USB wireless, and the speed went from about 4 Mbps to 32 by just changing from one port (where before I got around 35) to one just below it on the front of the desktop.
Check periodically https://www.speedtest.net