Ask HN: Frameworks to write cross-browser plugins and/or GMail extensions?

After reading about the Rapportive browser plugin/addon/extension that inserts itself into GMail, it got me to thinking if anyone has any recommendations on how to build browser extensions for various browsers from (hopefully) a mostly-shared codebase. Rapportive was here - http://news.ycombinator.com/item?id=1652362 edit - changed "plugin" to "extension", after reading this: http://colonelpanic.net/2010/08/browser-plugins-vs-extensions-the-difference/