Only on specific sites, all sites or in the UI too?

(That is, can you copy and paste from the text on other sites, what about from the URL bar?)

If the former, check your extensions (Tools > Add-ons) as they may be overriding the shortcut. If the latter, it may be a system keyboard setting.

Thanks for your reply. The copy and paste shortcuts don't work on any websites when using firefox, however they work fine in safari and all my other applications. Its the strangest thing. If it was a keyboard setting, wouldn't it apply to all applications?

I've tried on a couple different 10.7 machines and it seems to work for me on most sites but there are definitely a few where it's broken. What extensions are you using? (Check Tools > Add-ons)

Many thanks. I only had one extension - it was for craig's list. Everything was working fine on my laptop which didn't have this extension so I deleted the craig's list extension on my iMac and it now works perfectly.

Thanks again - I never would have known to look into the extensions.

Much appreciated!