Obviously plugin has to be enabled when building IcedTea. Apparently some systems (like iLo) can't use java web start with OpenJDK. For them applets have to be used but by default they won't open in Firefox - for iLo to work without surprises it is necessary to choose "always activate" for java plugin (Tools > Add-ons > Plugins, then find IcedTea-Web and choose "Always Activate" for it).
