(I hadn't realised this posted to a forum, I assumed it would it be sent to Mozilla so I only kept the text to a minimum)

Your crashreport is:
https://crash-stats.mozilla.com/report/index/bp-f3b6f25a-dcdf-4340-b3f1-341452111103
Firefox 7.0.1 Crash Report [@ _purecall | nsXPTCStubBase::QueryInterface(nsID const&, void**) ]
This may be related to a program called Trusteer Rapport. Which version (build ID) of rapport do you have? Does the crashing stop when you disable rapport?

Thank you! I would never have figured out Rapport was causing the problem. I have uninstalled it for the moment and that now allows Firefox to open. It looks like a problem with Rapport as it won't reinstall, but at least I now know to go to Trusteer's support to find out more. Thank you for your help, it is really appreciated!

Please report back to this forum if you have any results from the Trusteer helpdesk. Do you know which build ID you have? We have seen other crashes with a different signature which were solved by updating to build 1108.47 if I remember well.

I'm not sure the build ID as I've now uninstalled Rapport - but I have sent the Rapport Logs to Trusteer who have said they will have a look at them. I'll post back here when I have more info

Thank you.
The problem has been reported to bugzilla. It is found in version 11.08.52.
See Bug 699776 - crash in nsXPTCStubBase::QueryInterface involving RapportTanzan7.dll in the stack.

I don't have trusteer rapport on my computer but it will NOT open either normally or in safe mode. Crashes instantly since 8.0 update. Windows XP

In reply to my original problem; I have now heard back from the Trusteer Rapport team and they have assured me that the issue I have encountered has been fixed in the latest update of their software. I've now installed this and so far it seems to be working fine :)