I've been searching for a while to see if someone has gotten Google Gears working in Prism, but from what I've seen, it hasn't been done before. So I decided to see what it would take to get Prism to accept Gears as a extension.
My first stop was to Mozilla's Prism Extensions page: http://developer.mozilla.org/en/Prism/Extensions I ran into it a bunch of times before, but never paid much attention to it. There is one section that caught my attention "Porting an Existing Extension". This section tells you that you have to add this code to the install.rdf file to say that the extension works in prism:
<em:targetApplication> <!– Prism –> <Description> <em:id>firstname.lastname@example.org</em:id> <em:minVersion>0.8</em:minVersion> <em:maxVersion>1.0.0.*</em:maxVersion> </Description> </em:targetApplication>
So my next step was to get the actual Gears extension file: http://gears.google.com/ For some reason this was hard for me, but all you have to do is click on the link for your operating system below the big blue button, then the big "Install Gears" button.
Now you have everything you need to try to get Gears to work in Prism. Just open the .xpi, edit install.rdf to include the snippet above (I had problems adding in the snippet, but it worked for me when I substituted it with the targetApplication that was already in the install.rds). Now the extension should be good to add to Prism.
Here is a copy of the Gears.xpi that I created (it is Linux and Prism only). This technique should work with most Firefox extensions, but don't be surprised if it doesn't.