Mmmm, a problem has appeared. The program conflicts somehow with KeePass. I usually type my password for Yahoo Mail! manually, for various reasons (the most important being that I don't have KeePass in my PenDrive, so no access to the database outside home), but I decided to input it in KeePass to make things easier, and then the bug made its appearance.
Essentially, I can't log in to Yahoo!, because there is some weird minimizing and maximizing going on between the two windows (Firefox and KeePass), that prevent all details from being sent to the login page. After trying various times, searching around the KeePass help, and bashing my head against the wall, I remembered MiniMe, I closed it and... now works! Never encountered this problem with Opera, because I only run MiniMe in certain occasions, though it's also affected, albeit in a minor way (I can log in, but I can't see the login process).
Oh well, time to whip up another shortcut, I guess I'll use key combinations. Just to let people know if something strange happens with their windows.