Sorry, Google has stopped development of the Gears plug-in, and will not update it to support newer browsers like Firefox 4. They have announced plans to eventually migrate their sites from Gears to web standards like HTML5, but they haven't completed that work yet:

http://gearsblog.blogspot.com/2010/02/hello-html5.html

If you want offline access to your Gmail account, without using an older browser just for Gears support, you could try using an IMAP mail reader like Mozilla Thunderbird:

http://www.getthunderbird.com/

is there any way to revert to an earlier version of firefox so i can use gmail offline mal again? i live in a country with sporadic email access, and offline mail is very valuable to me. thanks anyone for any help on this.