Firefox 43 requires that extensions be reviewed and signed by the Mozilla Add-ons team and some publishers are still working through that (or in some cases, have no plans to do so). This article has more background: [[Add-on signing in Firefox]]. If necessary, you can make a temporary exception to the signing requirement -- this is global for all extensions, there's currently no way to only make an exception for specific extensions. (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (2) In the search box above the list, type or paste '''xpin''' and pause while the list is filtered (3) Double-click the '''xpinstall.signatures.required''' preference to switch it from true to false -- note that this is for '''Firefox 43 only'''