Well you'll need to do what I outlined above to realign the permissions for opening those files to an app other than Stuffit. As for the save versus open problem, that's more difficult. On Windows when you open a file in this way it saves it to a temporary folder and then opens it form there. On OSX that folder is your default save folder, which is also the Desktop in your case. There's no real way of getting around having to download something before you open it, however you can change the default save folder in Firefox (or any other browser) to somewhere out-of-site if it pleases you. Empty this folder every now and then to clear out all the old downloaded crap. Personally, I just let 'em come to the Desktop and then delete them from there when necessary. 🙂