Update Your Firefox Extensions to Firefox 3

In order to make your Firefox add-ons & Extensions to work on Firefox 3 you need to follow following simple steps.Here is the trick from Firefox developers.

Step 1: Update the install manifest

The first step -- and, for most extensions, the only one that will be needed -- is to update the install manifest file, install.rdf, to indicate compatibility with Firefox 3.

Simply find the line indicating the maximum compatible version of Firefox (which, for Firefox 2, might look like this):

2.0.*

Change it to indicate compatibility with Firefox 3:

3.0.*

Then reinstall your extension.

Note that Firefox 3 does away with the extra ".0" in the version number, so instead of using "3.0.0.*, you only need to use "3.0.*".
Note: Note that at this point more changes in Firefox 3 are expected. These changes may break some extensions, so you shouldn't release an extension with 3.0.* maxVersion to the users until the Firefox 3 release candidate is out. During the Firefox 3 Beta period, you should use 3.0b5 as your maxVersion.

There have been (and will continue to be) a number of API changes that will likely break some extensions. We're still working on compiling a complete list of these changes.
Note: If your extension still uses an Install.js script instead of an install manifest, you need to make the transition to an install manifest now. Firefox 3 no longer supports install.js scripts in XPI files.

Step 2: Ensure you are providing secure updates

If you are hosting addons yourself and not on a secure add-on hosting provider like addons.mozilla.org then you must provide a secure method of updating your add-on. This will either involve hosting your updates on an SSL website, or using cryptographic keys to sign the update information.

Read the rest at

http://developer.mozilla.org/docs/Updating_extensions_for_Firefox_3