Could you try again in case it was a temporary glitch? https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/ If you get the same error, I wonder whether some other software on your system is blocking it. Are you able to download other add-ons from the same site? When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site. 1. Clear Firefox's Cache orange Firefox button ''or'' Tools menu > Options > Advanced On the Network mini-tab > Offline Storage : "Clear Now" 2. If needed, delete the site's cookies here (this will log you out) While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies" Then try reloading the page. Does that help?