Not sure about this but sounds like you set Firefox as your default browser and Windows is trying to find it. If so you need to set another browser as the default. Also I suggest that all remains of Firefox be removed from the registry and any other place left on your hard drive.
Like you I tried Firefox and did not like the way it handled so returned to Mozilla 1.7.2 which I am using for the time being. Real satisfied with its handling of cookies.
I tried the firefox browser and found it wasn't for me and I have un-installed it.Now with internet ex.
However when I want to open a short cut I have placed on my desk top I get a dialogue box saying
Windows cannot find FIREFOX.EXE
Type in the excutable file to be used instead.
I have to say I'm clueless anyone any ideas ?