Firebug has a couple of versions which are available for download at any one time and from different sources. getfirebug.com and addons.mozilla.org. The getfirebug.com site has an explanation about the four different release streams.
Four Different Firebug Release StreamsThe stable release is on addons.mozilla.org and is linked from the front page. If you install from
addons.mozilla.org, you will be updated only when a new stable release is available. The latest release is on
getfirebug.com(this page). If you install from this site you will be updated incrementally to the latest usable versions:
1.2.0, 1.2.1b1, 1.2.1b2, 1.3b1, .... If you want to get on a early release stream, install the alpha versions. These will update as we make progress,
1.3.0a1, 1.3.0a2, ..., 1.3.0b1,.... Finally Firebug Lite for non-Firefox browsers is distributed as files or bookmarklets.