From the Firefox add-on page: NOTE: While the WebExtension version of this add-on has been released, it does not do much on Firefox 57 yet due to missing APIs. The new APIs are available for Firefox 58, but an accompanying middleware update must be installed first from eid.belgium.be. As of November 23rd, this middleware update exists for some, but not all platforms, as a beta release. Please check that before trying out this updated add-on. For Firefox 58, at least the following versions of the middleware are required to support this add-on: Windows: 4.3.2 Linux: 4.3.3 macOS: 4.3.5 I am currently using the 9999 ebuild (which is at v4.3.5-68-gab50b853 for the moment, which works fine)