Firefox Not Accepting Passwords

I have Firefox 3.0.10 and Firefox is not accepting passwords that I get from pop up boxes on specific sites. I belong to 3 different sites that require password log in information through a pop up box request (not actually embedded in the site)

On one site I have to type my log in and password twice before my browser will accept it. On the other websites the log in information has worked for a few days and then my browser no longer accepts the information and continuously prompts me with the log in/password box each time I type the information in. I am able to navigate to the support pages of these websites and request my log in and password information so I know that the information I'm typing in is correct.

Any ideas? I have tried clearing my cache and my authenticated sessions. Thanks for your help.