When visiting mozilla's ftp site (ftp://ftp.mozilla.org/pub/mozilla/nightly/) to download nightlies, additional testing build folders are listed at the bottom of the list: latest-OTIS/; latest-j4_client_mk_test/: latest-statictrunk/; etc.
Unfortunately these folders do not give information about what experimental feauture is being tested. I suggest adding a readme file to each of these folders with general information about the particular build. It will alow for potential testers to know what they should be looking for or observing when they install the experimental builds. I am sure doing that will take just a few minutes. RV
