Ok, I finally found it.
In "Internet Options > Programs" there is the option to "Re-set web settings"
(This is a slightly different layout than the picture above, but does the same thing)
This then reverts the opening back to IE. (In my case this is IE6)
Then to revert back to FF, I chose the Firefox safe mode, then under "Tools > Options > Advanced" chose the option to "Always check to see if Firefox is the default browser on startup"
It seems a bit of a rigmarole if you needed to do it very often, which fortunatley I hope I don't have to.
I do have the IE View Add-on option but for some reason it would not allow me to do what I needed to.