The crash reports indicates the crashes may be caused by a file named mcbrwctl.dll, that file is part of McAfee Site Advisor. Try disabling the McAfee Site Advisor to see if that stops the crashes. To do that, in the Tools menu select Add-ons to open the add-ons manager. Select the Extensions panel, then in the list of extensions select McAfee Site Advisor then click on the Disable button. Re-start Firefox to complete the process. For info McAfee Site Advisor has caused various problems with Firefox for quite a few people.