Which browser are you using to download Firefox? Does the error occur when you go to save the download, or after it's saved and you start running the installer? You might try the "full" installer from the following page instead of the quick-downloading "stub" installer (scroll down to your preferred language): https://www.mozilla.org/firefox/all/ Does that one work any better? If not, could you note the exact language of the error message so we can search past threads for it? Thanks.