I am running Firefox 4 beta for testing, and unfortunately Google hasn't updated their Gears extension for Firefox 4. At this point, it even seems that Google might not update Gears for Firefox 4. Do you guys know of any trick to get Gears to run on Firefox 4?
(For reference, here is where you can download the OS X XPI for Gears.)
Google have explicitly deprecated Gears; they no longer support it (as of about a year ago!).
They have switched to focus on HTML5 and other related new browser features. In effect they're saying that all the functionality provided by Gears can be done using HTML5.
Here's a link to an article about them dropping it (but there's plenty of others if you google for it):
http://latimesblogs.latimes.com/technology/2009/11/google-gears.html
The down-side is that not all browsers do support all the HTML5 features that are needed to replicate the Gears functionality, so by deprecating it they've cut off support for some browsers. Chrome is fine, of course! But the other browsers are catching up - certainly Firefox 4 should be fine.