The Add-ons Blog is now featuring a blog post on Firefox 7 compatibility:
It's important to note the changes to the JSON parsing interfaces. In a
nutshell the nsIJSON component no longer has the encode and decode
methods, and instead you should be using the global JSON object instead.
It's possible the change will be rolled back, but it would only be done
as a temporary measure. We strongly recommend that you change your code
accordingly to use the JSON object.
The post also includes a few notes regarding Firefox 8.
If you have any questions, please don't hesitate to contact us.
Add-ons Developer Relations Lead