Check Tools > Options > General > Startup: "When Firefox Starts": "Show my home page" | "Show a blank page"

As I said, I've already set Firefox to show a blank page on startup, but it insists on showing me a website

It's possible that some rogue add-on or bit of malware is doing this. Does the same thing happen when you start in Safe Mode?

Gerv

Make sure that you close Firefox properly, otherwise session restore will try to open the tabs and windows from the previous session.

See http://kb.mozillazine.org/Firefox_hangs and Firefox hangs or is not responding - How to fix (Hang at exit)

Set the pref browser.sessionstore.max_resumed_crashes to 0 on the about:config page to get the about:sessionrestore page immediately with the first restart after a crash has occurred or the Task Manager was used to close Firefox.
That will allow you to deselect the tab(s) that you do not want to reopen, but will allow to reopen the other tabs.

See:

• https://wiki.mozilla.org/Session_Restore#Preferences
• http://kb.mozillazine.org/Browser.sessionstore.max_resumed_crashes

There is nothing that can be done about this problem, since updating to Firefox 6.0.1, Firefox always opens in a Firefox page. You can do what you like on the options/general page concerning 'home page' and 'when Firefox starts' and it will have no effect what so ever.