All milestones released after M18 was labeled Mozilla 0.7, which would eventually reach Mozilla 1.0 and beyond. More about this is in the newly revised Mozilla development roadmap . You have two options, a nightly build or a milestone. New nightlies are released almost everyday, sometimes more often than that. Milestones are released when ready, which varies between five to six weeks apart, so they get outdated quickly, but are usually more consistent in their quality and stability.
Milestones are the results of work done on previous nightlies.
If you want to see Mozilla as it develops, would not mind that occasionally it might not work, experience some really nasty bugs on the occasion, or want to help report and test for bugs, you should download a nightly build. If you just want something that works, a milestone might be your best bet, although nightlies work very well the majority of the time. If you decide to try nightly builds, MozillaZine has Build Comments that regular post the progress of each nightly to help others decide if a recent nightly is worth the download. Builds with installer in its filename include an installer to guide the installation. Other builds require that you unzip or unstuff the file yourself.