I'm still having problems loading pages with Firefox - if I click on the refresh button it will load the page correctly, but otherwise only the address of the new page will show up at the top, while the content will remain from the previous page.

You may be running out of memory, possibly due to a Firefox memory leak. Try Ctrl-Alt-Delete on Windows and click on Task Manager, then click on the Processes tab and see how much memory Firefox is using. If it's much higher than 150,000 then the browser could be sluggish.

Or it could be one of his FF addons is causing the big spike on firefox memory.

NoirFan have you tried restarting firefox with addons disabled?

Click on Firefox button>>>Help>>>Restart With Addons Disabled