hi dja014, due to some new codesigning requirements that microsoft is requiring in 2016 (but which are incompatible with older windows versions), the regular installer for firefox devedition won't work on xp.

please download https://download.mozilla.org/?product=Firefox-aurora-sha1&os=win&lang=en-US instead & update it to the latest version. sorry for the inconvenience!

The Aurora aka Developer Edition is the name of one of four development channels. Release < Beta (b#) < Aurora (a2) < Nightly (a1)

It is not a separate product or Release.

Note the Aurora and Nightly gets checkins almost everyday so it gets updates each of those days.