Comment 2 for bug 297602

Does this only happen when there's multimedia content in one of the tabs or when you are playing a movie while surfing? If so it might be a known gstreamer issue. A temporary workaround is to open a terminal window (Applications menu, Accessories - Terminal). Type in

gstreamer-properties

select the video tab and change the Output drop-down listing to the option that says '...(no Xv)'

If that doesnt fix the problem you may want to reset that setting to its previous value, usually 'Auto'. Good luck fixing and happy tinkering :)