This reminds me of a problem one user had where .jpeg was tacked on to all .jpg files. I think the fastest workaround would be to remove Firefox's accumulated download action data and go back to default download settings. This would be like resetting everything on the following tabs, but also would reset things you can't see there: orange Firefox button (or Tools menu) > Options > Applications 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 (in case you need to mine it for data or restore it). Restart Firefox. Do zip files download normally again?