Re: "The long term plan is to migrate our code to web technologies"
Web technologies in this case doesn't mean things are moving onto the web. It means more use of technologies that are also found on the web.
Specifically, Firefox has always used a lot of XUL (https://en.wikipedia.org/wiki/XUL) but is gradually phasing it out because they are running into its limitations. Thunderbird is also a heavy user of XUL and because they use a lot of Firefox code will need to find an alternative too. In this case we'll probably see XUL replaced with "web technologies" such as HTML5, CSS3 and JS. That doesn't mean they will run in the cloud, they can just as easily be used locally.