I found it!
I installed the mouseware software, otherwise my thumb button isn't recognized and I use that for "minimize all" (aka boss button...). Default, that software uses the "standard scrollwheel functionality" for, well, the scroll wheel (makes sense, right?) I put that to "standard third button functionality" and now it works in FF2 too (I got FF3 installed on a virtual pc where I didn't install the mouseware).
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