I just published a post to the add-ons blog with some advice on how to
approach supporting Firefox 10 ESR users.
Tl;DR: use SDK 1.5 to package for Firefox 10 ESR.
the rest of the post is here:
http://blog.mozilla.com/addons/?p=4467
We're interested to have your feedback on this issue, in this thread.
I'm also curious to hear from you if you are an add-on developer who is
intending to do a lot of ESR targeted add-ons.
regards, Jeff