graeme_p - 9:28 am on Jan 13, 2012 (gmt 0)
Could you not keep old versions in VMs with auto-updates turned off just for testing? That is what I used to do for IE. Keep a copy of the VM just after each upgrade, and on the next upgrade it becomes your previous version VM.
Usage of every FF version except the current stable, the last stable, and 3.6 seems very low, and with minor differences you surely will not need to do a lot of testing of each version.
Having an extended support release should help, as people who do not want updates will use that, while those who you the latest stable will probably enable upgrades - so almost everyone should be using either the latest stable version or the extended support version.
It will be interesting to see which Linux distros decide to go with as the default, especially if, like Ubuntu, they have their own extended support and frequently upgraded versions as well.