Firefox tries to download file on every startup

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http://ubuntuforums.org – I downloaded a .deb from Dropbox website. When I tried to open it with Software Center, I got an error about unresolvable dependencies. Anyway, I tried again later and it worked properly. But now, every single time I launch Firefox, it pops up the dialog saying "You have chosed to download file.extension, what would you like to do?" or some such wording. So far, I've tried deleting everything from my Downloads folder, and wiping my Firefox history. What else could I try? Posting here because I'm using Precise Beta1. (Hardware)