I am upgraded to Firefox 27.0 a few days ago but noticed it was sluggish at opening windows/new windows. I have it running on OSX 8.5 on an iMac with 12gb dram (inc from 6gb recently) and have selected -1 for memory caching and disabled disk caching. I selectively turned my extensions off and isolated it to the selfdestructing cookies add on, which I have now disabled and speed is back to normal. I had another problem where some sites couldn't be accessed as though the server wasn't working, although present on an engine search, its cached page could not be accessed either. I didn't think much to it until I tried to access DVLA on the www.gov.uk site. I traced this down to AVAST web shield. In Preferences Advanced turn Scan secured connections off. This restored normal functionality.