Before you jump straight to the comments section, yes I’m aware that Firefox 4 beta doesn’t have to be compatible with pre-existing add-ons. The idea of my experiment was to see how much Firefox’s new add-ons center changes the way browser extensions work. The answer is: a lot!
Hacking Firefox to make it run addons designed to run in previous versions of the browser is hardly an advanced job. Just add a couple of Boolean values to the advanced preferences and you can tell Minefield (the best name for a beta program ever!), not to check add-on compatibility. Generally, this can force compatibility when the version change is minor (e.g. 3.5 to 3.6). It doesn’t usually work for most add-ons across major version updates.
And 4.0 is no exception! After telling Minefield (4.0b2Pre) to accept all my 3.6 add-ons and restarting, the extensions all appeared to have loaded satisfactorily. FoxyTunes, Alexa, Google Wave, Tumblr Post, Firebug and Echofon all appear in the status bar where I’d expect to see them. But none of them do anything. Nor does Greasemonkey. In fact, except for the built-in extensions, the only add-on currently seems to function is the classic Tab Mix Plus.
So for those of you asking the question: ‘how do I force an add-on to be compatible with Firefox 4.0?‘, the answer is: You can’t. Be patient, Firefox 4 will be released soon and your favorite add-ons will be updated in no time.