Did what you said via the Start menu, as I open FF via Rocketdock. It was set at Normal, so I changed it to maximised and it hasn't made any difference.
I noticed yesterday, it did the same thing on the laptop as well.
We have a little support section on that particular site, so I have posted it in there to see if anyone else is having the same problem.
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Dell Inspiron 1720 running Vista Home premium SP1, Firefox 3.0.4