there are two tabs in the speech window, <em>speech recognition</em> is the one you are referring too, the other is <em>text to speech, </em>that's the one me thinks... It might be different if you are running leopard. I tried to simulate your problem myself, couldn't do it, I'm running tiger.
The bookmark thing, either, control+click the bookmark > delete (actually, this only works for the bookmarks in the toolbar of the browers window) <em>or</em> in the Firefox toolbar under bookmarks > organize bookmarks.