First thing to think about is if that older PC will handle Win7 or not.
Ask DNS support about whether their program is compatible with Firefox 6 or not.

Thanks for response! Have been told older PC can handle Win 7 but will verify. DNS support said Firefox 6 may or may not be compatible as it has not been tested which I don't find particularly helpful!

AFAIK, it's been over three years since a WinXP laptop or desktop PC has been sold; I specifically bought a laptop in June 2008 because it was being clearanced by that retail chain to meet Microsoft's deadline, and got it real cheap. It was tagged for the store manager to purchase, but he let me have it because I exceeded his expectations for the services I performed for his store and thought I deserved it. Anything that wasn't made for Vista-compatibility will be very iffy with Win7, IMO - so that basically rules out anything that was made before like Oct or Nov 2007.

Unless another DNS 11 user happens to see this thread and can verify whether it works in Firefox 6.0 or not, I don't what to recommend except for you doing a web search to see what other DNS / Firefox users have to say about compatibility.

P.S. PC is a Dell about 10 years old

Nope, I wouldn't even consider a 10 year PC as a viable candidate for either Vista or Win7, sorry.