Can't use multiple select with admin !?!?

I have a strange behavior that I didn't notice since now. I can't use the multiple select fields (taxonomy, subscriptions ...) every where on my site. I just can't select anything like if it was disabled. But its true only when i'm logged in as admin, actually all users with admin access (all access). Normal user can select multiple vocab/terms. Stranger behavior is that apply only with Firefox !?!?!?

I've searched through the forum but didn't found any relevant topic. I've tried on another computer with firefox, niet. I've looked in the params of firefox but ...

My setup is :
WinXP sp2
Firefox 2.0.0.11
Drupal 5.6

Thanks for any help

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