Just a quick addendum to the earlier post: New AMO Launched. Basil Hashem who is part of the AMO Team added a comment in reference to the inability to install incompatible add-ons:
Try visiting the “Advanced Details” section and take a look at the Complete Version History link – it includes all available versions and doesn’t do the smart detection so that testers, developers and others can download any version of an add-on (for whatever OS platform and edition of Firefox) you want without the hassle.
Here is an example:
Now I can understand the justification for keeping this section collapsed by default, since everyday Firefox users really won’t have a need to go here. There is also a link to the add-on Developer’s Home Page in this section as well.