Issue-6755: Additional Firefox versions for Build Addons (#864)
Issue-6755: Additional Firefox versions for Build Addons (#864)

* Added functionality for downloading developer and nightly firefox versions

kevmoo/travis-build,andyli/travis-build,craigcitro/travis-build,andyli/travis-build,JuliaCI/travis-build,kevmoo/travis-build,craigcitro/travis-build,JuliaCI/travis-build