firefox + webclient : delayed keystroke arrowskeys?

Hi,

lately I experience some problems with the webclient.

while replying to zimbra messages, the 'directional' keys like home, end and arrows keys work only after a second delay.
This make the use of especially the arrows key difficult.

But, when starting a new message, those directional keys works as expected.
Likewise while entering this post, the keys react instantly.

recently Firefox upgraded to v4.0.1 - could this be the cause?

Suggestions welcome
glenn