Just debugged a little issue with checkboxes on web forms in Visual Studio 2008 while using Firefox. I have a column of checkboxes that were cloned from a single instance (cut/paste in the text portion of the design view) then distributed vertically by manually editing the top value.
The checkboxes worked fine in IE, but in Firefox I could not get them to change state to Checked. After changing to a unique z index the checkboxes started working again. Interesting.