Rewriting ancient code

As a counter point, the concerns of a mail client are often of more reduced scope than those of a browser. The security ones are the same (do HTML emails allow JS? I don't know, but if they do you're running potentially untrusted code), so sandboxing does justify the same degree of effort. However, you don't need to be so strict, for example, with performance. Honest emails tend to be rather lightweight compared to full fledged websites. So, maintaining Gecko (or even using a simpler engine, but that only applies to rewrites and the like) may take less effort because you're not making all that parallelization work and what not that Firefox desperately needs. You just need to fix bugs that appear and probably keep CSS up to date or stuff like that (which is obviously non-trivial anyway).