Many sites (for instance aula.au.dk) sends binary files using the application/octet-stream mime type. This is annoying for Firefox users, as they presented with a dialog asking Would you like to save this file? instead of showing the standard dialog offering both the options Open with and Save file.

Bug 327323 has a patch that uses gvfs to determine the mime type from the extension if Firefox cannot determine it itself. However, if you can’t wait for that fix to reach your distributions package repository, here is a patch for Firefox 3.5 that bypasses the security restriction for octet-streams and executables. Note that it won’t remember the application you specify to handle the file, but your can circumvent that by using the OpenDownload add-on, which uses your native mime system to look up the default application.

Edit: I found out that OpenDownload also alters the security restricted download dialog, so you can suffice to install OpenDownload if you want to avoid patching Firefox. You will only be able to open the file with your OS’ default handler, though.