[mikeratcliffe]

Our devtools have unit tests that run for every tiny change that is made to Firefox. Building the tools in gives much more stability. We are also using and exposing a lot of low level Firefox stuff and fixing bugs as a result.

So in short you get a better, more extensible browser, more stability and the chance to take a look at how something you use every day works.

It's a win-win in my opinion.