Two easy fixes:
1. Stop using the less-secure "master password" feature.
OR
2. If unwilling to do (1), browse your saved passwords and delete the one for the forum index page. Save the password for the login page. When you need to login, visit the login page. Oops, doesn't work.
2. Stop using Firefox, and start using
Opera.
Or mandrav could probably change some SMF setting somewhere, I guess...it's not like the login box on every page is a huge bonus or anything...