I had posted a few days earlier after FF 56.0 installed on my Win 7 Ultimate desktop (home built Asus Sabertooth Z77). Symptoms were slow response of mouse movement while in FF, delayed display of typed characters in FF, and very slow page display. I initially thought it was 56.0 and rolled back to 55.0.3 which was working fine. No luck. Now 55.0.3 was showing same symptoms. Uninstalled all versions of FF and re-installed FF 56.0 with a new profile. Problem seemed resolved but reappeared next day after 56.0.1 update. Am using IE 11 to write this and am not experiencing any lag.
Initially using FF, I brought up task manager and found two Firefox.exe's running. Killed one and it didn't kill my FF session. Also noted that FF is using 12-15% CPU and a good chunk of RAM. Now I'm pretty much convinced I have something on this machine that I do not want. FYI, I have a Win 10 laptop running FF 56.0 with no performance issues at all. Both are 64bit installs.