Hmm - OK, done another test
Ran the firefox installer again, no clean up just run installer.
Installer run's OK, takes about a minute. As it is install over existing, all extensions, bookmarks maintained......
Fire up firefox in sandbox - and now it is working.
Did a test prior to this, to confirm it was not running and was looking at network connections and whilst it looked like firefox was trying to connect to a site there was something else stoppping it. However, once I ran the installer again, it is working.
This would seemingly rule out any issue with the extensions.
I will have to run some Sysinternals tools to see what is happening here