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):
2.0.0.*
Change it to indicate compatibility with Firefox 3:
3.0.*
Then reinstall your extension.