It seems Firefox version numbering has suddenly accelerated to resemble that of Google Chrome.
Currently the Mozilla website offers for download the latest version 6.0.2, with the "Older Downloads" section containing only version 3.6.22, with this note:
Firefox 3.6.x will be maintained with security and stability updates for a short amount of time.
As web developer who would like to make sure some applications and sites also work with Firefox 4 and 5, what would be a good place (as official as possible) to get those versions from?