Try starting Firefox in safe mode in case that's some broken extension:
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-is...
Re:Firefox 57.0 Released
by __BrianDGLS__ on Tuesday November 14, 2017 @10:37AM
(#15695898)
No extensions installed. Literally just downloaded firefox.
Sibling comment recommended opening a bug. I concur: https://bugzilla.mozilla.org/enter_bug.cgi?format=guided#h=b...
Re:Firefox 57.0 Released
by TheRealPomax on Tuesday November 14, 2017 @11:16AM
(#15696319)
sounds like you should file a bug about that so they can be aware of that problem and look into it.
I've had to open task manager twice now to kill it. Only downloaded it 10 minutes ago.