As you probably know, Mozilla has switched to a rapid release development cycle, planning to release Firefox 5, 6 and 7 by the end of the year and they've released Firefox Beta (currently Firefox 5), Aurora (Firefox 6) and Nightly (Firefox 7) channels from where builds are gradually promoted until they become the new stable Firefox. See this for more info.

We had an official Firefox Nightly channel PPA and Firefox Beta channel PPA for some time and starting a few days ago, there's also a Firefox Aurora Channel PPA, making it easy for Ubuntu users to install any Firefox development build.