On Windows 7 Pro, Firefox 3.5.5 this kills most of my other Browser extensions. I ordinarily run Wordreference Translator, Gtranslate, Addblock Plus and Xmarks. When your Wolfram extension is installed, the context menu for my translate extensions goes blank, and when I try to disable your Wolfram extension, the browser crashes completely requiring a computer restart to restore it.
On the other hand, the extension is really nice, and integrates pretty smoothly into Google results. I like it, you should just make it play nicer with others.