messing with a chromium install (it's good for pages FF barfs on) and it seems impossible to disable middle-click autoscroll while still allowing mouse gestures via middle click Afficher plus
update, this little userscript actually works, disabling mid-click autoscroll in chromium while still allowing gesture add-ons to use that button
http://www.enhgo.com/snippet/javascript/disable-auto-scrollingjs_jackm_javascript