If you are running the relatively new XenialPup 7.5 you'll probably want to update the Pale Moon browser. The 27.8.3 release appears to fix a problem that I would describe as a "memory leak" where, if the browser is left running, processor utilization over the course of several hours approaches 100% and the machine becomes unresponsive.
http://www.palemoon.org/releasenotes.shtml