While you may think that one should be able to browse to addons.mozilla.com and download any old version of a plugin, that would be too simple and user friendly. Instead Mozilla has decided to prevent users from installing, or even downloading, versions deemed incompatible with the user's current version of Firefox.
Find the "Works with Firefox " text next to the plugin version you are interested in and use a user agent
that falls between the supported versions. For firefox 4 you could use
Mozilla/5.0 (Windows; U;
Windows NT 5.1; en-US; rv:2.0.1) Gecko/20110606 Firefox/4.0.1 (from
and the green buttons appear!
If you need to bypass firefox addon compatibility checking too, Nightly Tester Tools is one way to do it. Install it and check the Tools -> Nightly Tester Tools -> Force Addon Compatibility option.