If anyone wants to take this, I'm betting the issue is in base/firefox/module.cc 
where we try to detect whether to load the our module.

I tested this with the latest version of Prism (0.9.1) and Gears (0.5.4.2). I was
able to sync all my mails in Gmail to the offline store with no issues. Seems to work
great.

I'm attaching a patch for install.rdf.m4 with what I think are the correct Prism
version numbers.

The one issue I did notice is that Gears creates a desktop shortcut for Gmail when
offline support is turned on. This doesn't make sense for Prism since there is
already a shortcut (and the new one doesn't work). I can try to take a look and
propose a patch for this (checking whether the web client is Firefox or Prism),
especially if someone can point me to the right file in the Gears sources.

gears4prism.patch 624 bytes   Download

Did you test on Windows?

Just tested on Windows and it works great. It's a bit trickier to load it in Prism
since you have the hack the registry, but the install.rdf change is a step in the
right direction.

Matthew, what is that gears4prism.patch?  If I just dl and run it, will it make gears
work with prism?  If it requires hacking, can I get layman instructions?  I want to
use gears and prism for Gmail, Gdocs, and Scrybe (but I don't know if it's possible
for Scrybe).  I have Winxp.

Now that Prism reached 1.0 beta stage is really great, I'm using it with GMail and
Google Calendar and offline support would significantly improve the experience.

In re: Gears adding a desktop shortcut...

You can opt not to do that once you give GMail permission to use Gears. I'm currently
using Gears for my regular GMail with the latest beta-release Prism under Windows
Vista, with just the modified install.rdf, and it works just fine. On a side note,
it's a shame that Google Apps doesn't support Gears just yet.

I tried installing the latest Gears (0.5.29.0;official;opt;win32;firefox) with the
latest Prism (1.0b1) with the above patch and instructions. It installs, but isn't
detected.

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.

This is of course a short circuit hack. Someone more familiar with the codebase and
APIs should add a clause checking Prism's Gecko version.

I'm having the same issue as above. Unfortunately, the hack doesn't work on the
latest latest Gears (0.5.30.0) on OS X w/ Prism 1.0b1 + Firefox 3.5. I tried having
stub.js return ff30 and ff35.

I'm having a similar issue. I installed Prism (1.0b1) and Gears (0.5.32) with some
modifications in indtall.rdf and stub.js. Gears as an extension was detected in Prism
for GMail. Unfortunately, when I continue to sync my mail, Gears report that
https://mail.google.com is add to denied list; where as I haven't done any such thing.
Weird...

Here is the hack for Prism (1.0b1) and Gears (0.5.32). In stub.js I was earlier
choosing ff35 instead of ff30. I works now..