I discovered that an add-on named Simple YouTube to MP3/MP4 Converter was causing FF to run extremely slow, e.g. I'd type text in, say, an email, and it would be 6-10 seconds before the text would display, and pages were painfully slow loading.
Another problem was that drag/dropping in a page with Dragula was slow and herky-jerky.
If you've got Simple YouTube to MP3/MP4 Converter running and you have problems like these, try disabling or removing it and see if that fixes things.
I'm running FF 56.0 64-bit in Win 10.