This can occur when Firefox has associated an unusual content-type like "application/download" with the .zip file extension. The '''handlers.json''' file which stores these associations is difficult to edit, but you can rename the file and have Firefox rebuild it. Here's how: Open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > "?" Help > Troubleshooting Information * (menu bar) Help > Troubleshooting Information * type or paste '''about:support''' in the address bar and press Enter In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows File Explorer. Leaving that window open, switch back to Firefox and Exit, either: * "3-bar" menu button > Exit * (menu bar) File > Exit Pause while Firefox finishes its cleanup, then rename '''handlers.json''' to something like '''handlersOLD.json'''. Start Firefox back up again. Did that fix it? Note: you may need to update a few downloading preferences again, for example, if you don't want PDFs to open in a tab in Firefox.