Mozilla has released the first update for Firefox 4. Firefox 4.0.1 comes with fixes for three security vulnerabilities, two of them labeled as critical, so you are encouraged to update as soon as possible.
Another 50 bugs have been fixed for the update as well, including one related to large PDF files (>5MB) not correctly loading.
To update, in the Firefox menu, select Help > About Firefox. Firefox will automatically look for and download the update.