YWMS was a plugin for JOSM which has now been superseded by recent improvements to the main WMS plugin. See the JOSM/Plugins/WMSPlugin for details of how to make that plugin work with Yahoo! Aerial Imagery.
Old YWMS information (Historical Interest Only) - Superseded by the WMS plugin
This information is provided for historical interest,
The YWMS plugin used to allow you to use Yahoo! Aerial Imagery as a background in JOSM. It used Firefox to "browse" the Yahoo imagery through the standard Yahoo APIs. Firefox then grabbed an image which was fed into JOSM through the WMS plugin
The plugin didn't work with Firefox version 3 and above This is the main reason that Yahoo support was added into the WMS plugin instead, making this plugin obsolete.