Hi,
I’m facing regular issues with this forum when i keep the tab open (using Firefox 56) : after a few minutes, the CPU load and RAM usage begin to increase. After 2 days, the process used 1.5Go RAM and 30% of my CPU (only for the forum tab).
I’m not a guy who closes tabs, so this is kind of very annoying…
I hope you didn’t install a bitcoin miner 
I’m using Firefox as well and have no performance problems as of lately. I think the forum software (discourse) uses stuff like web sockets to provide notifications etc… If you are sure of the performance problems the discourse forums would be a better place to report them.
My performance problems with Firefox stopped once I uninstalled the Ghostery plugin, btw. so selectively disabling some plugins (like ad-blockers) may also help track down the performance problem.