User Agent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20100101 Firefox/12.0 Build ID: 20120420145725 Steps to reproduce: Firefox 12 takes 100% CPU just after start with a blank window, then in 5 minutes again. It did not happen with previous versions. It is always reproducible even in Safe Mode. Steps: 1. Start Firefox with default blank page 2. Do not open any URL, just wait Actual results: When Firefox starts it takes 100% CPU for about 20 seconds. After 5 minutes it happens again. Using Process Monitor I found there are reads and writes of .sqlite+journal files (mostly urlclassifier3.sqlite). It happens everytime Firefox is started. The problem has started after update from version 11 to 12, on multiple computers. Expected results: No 100% CPU utilization.

Deleting urlclassifier.pset and urlclassifier3.sqlite files from profile helped to solve it.

is it perhaps one of these ? https://bugzilla.mozilla.org/buglist.cgi?query_format=advanced;resolution=---;short_desc_type=allwordssubstr;product=Core;product=Firefox;short_desc=classifier;list_id=3575495

(In reply to Wayne Mery (:wsmwk) from comment #2) > is it perhaps one of these ? > https://bugzilla.mozilla.org/buglist.cgi?query_format=advanced;resolution=--- > ;short_desc_type=allwordssubstr;product=Core;product=Firefox; > short_desc=classifier;list_id=3575495

Yes, it is close to bug 764802. Actually it is improved in FF 17.0.