Firefox versions 9 and 10 work fine for me as ThinApp packages.
For updating a ThinApp package, I suggest reviewing the following:
ThinApp Blogs: A Guide to Updating a ThinApp - http://blogs.vmware.com/thinapp/2010/05/thinapp-updating.html
ThinApp BootCamp Series - http://vmware.com/go/thinappbootcamp