The problem turns out to be the detection code in components/stub.js in the xpi.
Before creating the zip/xpi, if I change the function getLibFilename in
components/stub.js to return “ff30”, and *then* install the xpi in Prism, it works.
So all you need to do is to grab the Gears xpi file and let the getLibFilename function in components/stub.js directly in line 1 return “ff30” if you use Firefox 3.0 or return “ff35” if you use Firefox 3.5. And now you can install the xpi as any other Firefox extension. But it is important to edit the file before you install the extension, because otherwise it dose not work.
This is a hack, but since the issue is in the Gears bugtracker and it it set to accepted, we can hope that it will get fixed in the near future.