When Firefox displays the Open/Save/Cancel dialog for these problem downloads, does it indicate that it recognizes and understands the type of file it is? You can tell because the dialog will state the type of file and show a logical application to handle it. (Examples attached.) Most web servers send a specific content type that Firefox can find on its list of file handlers here: orange Firefox button (or Tools menu) > Options > Applications However, some servers only indicate that the file is a generic binary file, and Firefox doesn't actually save your instructions for that kind of file because the next time, it could be something completely different. That said, it's also possible for the settings file to simply become corrupted. You may need to reset those download handler preferences back to their defaults. Here's how. Open your current Firefox settings (AKA Firefox profile) folder using Help > Troubleshooting Information > "Show Folder" button Switch back to Firefox and Exit Pause while Firefox finishes its cleanup, then rename '''mimeTypes.rdf''' to something like mimeTypes.old (or you could delete the file). Restart Firefox. Hopefully now your download handler preferences will be remembered.