Go to C:\Documents and Settings\user\Application data\Mozilla\Firefox\Profiles and rename the profile in there. Start firefox. All your settings and bookmarks are gone. You can copy the bookmark.html from the old folder to get back the bookmarks.
Open real window instead of tab
Go to Tools->Settings->Tabs. Deselect to open window external instead of tab.
There might be a problem caused by an extensions.
Use Tools->Addons->Extensions to disable all extensions.