Firefox itself is quite fast [1]; if you're having issues, there's a good chance that it's due to an add-on [2]. A (historically; I'm unsure if this is still the case) particularly egregious offender [3] is AdBlock Plus, but µBlock [4] does come highly recommended.
Hmm...that doesn't sound like normal behavior; my uneducated guess would be that there's a specific webpage or extension with a leak. Next time it happens, it'd be fantastic if you could go to about:memory, save the reports, and file a bug with them attached:
Until suddenly the show stops and I have to kill it via Taskmgr to inject new life into it due to memory suffocation.