Are you using Ctrl+n to open the new Window? That usually will open your home page in the same mode you're in ("restored" or maximized). The page itself could have a resizing script, but that would be somewhat unusual.

No I'm starting Firefox from the taskbar shortcut, created via the installation program. The default homepage is "about:home" so unless there's a script on that page doing this then I don't know what.

I'm using Adblock+, Firegestures and a couple general use plugins.

I have had this problem too, after rebooting my computer, when Firefox tries to restore the previous session. Often many pages which I can only view maximized.