Try comparing all your startup setting with mine.
In FF type about:config and aggre to the warning.
In the filter, type startup
compare what you see in there with this:
http://www.rojash.co.uk/FF.png
Obviously, the first entry should point to your won choice of startup page.
Sorry AOG.
When diagnosing a software problem, the first thing I attempt to do is duplicate the fault. Well I've tried tweaking every likely value in the config settings and in the various ini files, and am still unable to duplicate it, so at the moment I'm out of ideas.