Thanks, 3rd user has same - forum thread is at
https://forum.openstreetmap.org/viewtopic.php?id=58784 but reports identical problems on a totally different Flash-used website
as well. So we may well need a Firefox and/or Flash fix. Using Chrome is
ok, so can work round it ok.
Just to confirm I'm seeing this problem too with Firefox 54.0 on Ubuntu
Linux, with about:plugins showing "Shockwave Flash 26.0 r0" and
https://helpx.adobe.com/flash-player.html reporting version 22.214.171.124.