When you develop a Firefox extension you’re required to specify the minimum and maximum versions of Firefox which your extension is known to work with. This is done by specifying a minVersion and maxVersion in the install.rdf file inside your plugin’s archive. Since my ClearSkyPlugin isn’t likely to break in any future version of Firefox, I attempted to bypass the version checking by setting the minVersion to 1.0, and the maxVersion to 9.0.
Of course, today Firefox 3.0 was released and my extension was detected as incompatible. After some twiddling I managed to figure out I had to set the minVersion to 1.0.* to get things to work.
If you’re having trouble getting your downloaded extensions to work, you might just disable extension version checking altogether.