Is this only certain sites, for example ones with lots of video content. You also need to consider what else your computer is doing, possibly as automated background tasks such as

• security updates or scans
• disk indexing
• downloads

You could try running with minimal extensions enabled and use Task Manager to check ( Ctrl+Alt+Del; or on newer windows machines Ctrl+Shift+Esc) what is running and how busy the CPU is.

See also

• Firefox uses too much memory or CPU resources - How to fix
• Basic troubleshooting

thanks for you reply john99 i don't think its that as all i am doing is checking email . i dont think it could cope with two things at once.

See also:

• http://kb.mozillazine.org/Firefox_hangs
• Firefox hangs or is not responding - How to fix

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the 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