I'm somewhat uncertain personally about the future of Mozilla myself, as well as compatibility issues and a lack of mindshare.

Also, I feel working with the Chromium codebase is easier if you only apply superficial changes, e.g., the linked browser. The patch files are all very simple, so the fact that Chrome is generally less crufty (Mozilla is working on cleaning up a lot of ancient stuff, which causes us a lot of pain but is probably great in the long term), simply due to being newer, might make it easier to get started. Although I always felt the most significant hurdle (if you know C++ and JavaScript sufficiently well to patch a Browser) is getting the stuff to build, Mozilla is doing reasonably well on that front. Building Firefox always felt less annoying than building Chromium.