Firefox doesn't integrate OSX's dictionary, so if you need one, here is an excellent extension for it. Quick TransLation (qtl) 10 supports firefox up to 3.0 beta 5 and most recent 3.0pre nightly. To install https://addons.mozilla.org/en-US/firefox/addon/5641 select terms, by double clicking or dragging select, (you can set it as selecting terms while pressing of "alt") bring up a movable floating penal with definitions. There are quite a few languages you can choose for the translation, select language by clicking the flag icon in status bar. options can be accessed by clicking that "b" in status bar, "quick mode" will display translation in urlbar.