One of your add-ons might be the culprit too. Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode with all add-ons disabled. If that solves the problem, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and possibly Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

You have the Porn4Mac malware in your plugins list. See [tiki-view_forum_thread.php?forumId=1&comments_parentId=279767]

tried it but after scanning it says that the DNSChanger trojan is not on my computer. i really don't know how i would have gotten "porn4mac". i understand the obvious reply, but i really do not visit those sites. any other suggestions?

You can see the installed plugins on the about:plugins page. If you set the pref plugin.expose_full_path to true on the about:config page then you see the full path of plugins on the about:plugins page. It is best not to leave that pref set to true as it exposes that full path to web servers, so reset that pref to false after you are done with the about:plugins page. You can access about: pages via the location bar, just like you open a website (about: is a protocol like http: and used for internal purposes).