Previously, Firefox was using the ISO-8859-1 character encoding for the username and password in Basic HTTP authentication requests. Firefox 59 and later will use UTF-8 instead for the
Authorization header to make sure non-ASCII characters like French accented letters are encoded properly.
If your site only allows alphanumeric and common symbols in user credentials, this change won’t be a problem. However, sites accepting non-ASCII usernames and/or passwords have to be updated to support both ISO-8859-1 and UTF-8, or users may have difficulty in signing in.
Since Google Chrome is already using UTF-8, popular application frameworks are apparently sniffing the
User-Agent header to determine which character encoding to expect from the browser, as noted in RFC 7617. The same interoperability approach may apply to Firefox 59 and later. Given that Firefox 52 Extended Support Release (ESR) will be available until August 2018, sniffing code should check not only the browser name but also the version number.
So far JBoss Enterprise Application Platform (EAP) is known to be affected by this change.