These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
Firefox 3.0b5 - 3.0pre, SeaMonkey 2.0a1pre - 2.0a1pre, Thunderbird 3.0a1 - 3.0a1
This version adds a more controllable method for overriding add-on compatibility. Instead of needing to use the indiscriminate built in application settings I recommend switching to using Nightly Tester Tools to give you the choice which add-ons you override the compatibility for.