Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

>In Firefox 4 Safe mode also disables plugins (Tools > Add-ons > Plugins) and hardware acceleration (Tools > Options > Advanced > General)

Do you have the NoScript extension installed?
It gives audio feedback when scripts are blocked on page loading. In NoScript Options look at the Notifications Tab, un-check to box for Audio feedback when scripts are blocked in the middle of that tab.

Thank you for your response. That does sound like it could be the problem as I get lots of notices about script not responding. I'll have to wait until someone with PC know how visits as I don't know how to find this, but I will look further into it to see if that's the problem.

Thanks again.

Script not responding messages tend to occur less frequently or not at all when you're using No Script, as it block scripts from running unless you allows the scripts for a particular domain.