Current, new (59.0.2) still locks up and "churns" (?) for 30-45 seconds
Prior to, and after updating to 59.0.2, after turning on PC in a new session, and using PC for3-15 minutes, Firefox locks up the system at 100% cpu time. It lasts for 30 sec to 2 minutes then goes back to normal. Typically, it only does it once. Nothing will interrupt it while churning.
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Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.
A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?
You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no
Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.