After about five minutes or so browsing with Firefox, it freezes unexpectedly, often trailing the window when I drag it, sometimes forcing me to reboot the system. It doesn't seem to matter whether I'm watching videos, viewing pictures, or reading text. I thought perhaps memory was being taken up by my panels, so I removed all my extra icons except for the ones that were used in the default setup and this doesn't seem to fix the problem. Any ideas?

Have you updated it recently?

Use apt-get or synaptic to update firefox and/or the whole system, if that doesn't work synaptic can remove and reinstall FF.

If it still doesn't work try another browser or look in the Ubuntu or Firefox forums chances are someone has seen the problem and fixed it already.