Strange behaviour in FF with Garland after uploading 3rd file

When creating a story and attaching files to it via upload module (with the 'attach' button), the fourth file gives a bit of a problem: the button 'Attach' becomes unclickable and the cursor turns into a small caret as if a textfield has been activated. One can still focus an other control then tab to the Attach button and activate it with the spacebar. I encounter this consitently on Firefox 2.0.0.1/WinXP with the Garland theme, not bluemarine.