I'm using the latest version of Firefox 1.5 & I too had problems but it seems as though the problems seem to eminate inpart or whole due to remnants of previous versions of Firefox left on your pc.
Click on the following link below & it explains what to do for various situations
http://forums.mozillazine.org/viewtopic.php?t=341279
Click on the following link below & it explains what to do for various situations
http://forums.mozillazine.org/viewtopic.php?t=341279