For security reasons, Java archive (JAR) files can no longer be loaded from remote locations by default. This change was originally made with Firefox 45 but soon reverted because IBM iNotes was broken. The product was then updated in May 2016 to not use JAR for Firefox, hence the re-attempt with Firefox 55.
iNotes customers must install the Fix Pack to solve the compatibility issue. If the instance cannot be updated now for some reason, use Firefox 52 Extended Support Release so the remote JAR support is available until
No other browsers support JAR. No other broken apps are reported so far.
Update The remote JAR support has been completely removed with Firefox 61.