Perhaps there's a preference panel that shows all the features of scroll-click, and on that panel you can specify what to do for normal text. For instance, in addition to opening URLS after clicking on it, it also can copy/paste highlighted text (which can be convenient, but also can get annoying if you're writing a paper and do it accidentally!) Furthermore, if you scroll-click on a tab in Firefox, it closes it.
EDIT: well, I can't find a setting in Firefox preferences for scroll-click, or a system preference through the GUIs... maybe it's some obscure command-line thing on linux.