Possibly taking one of the best parts of Firefox, the addons, and replacing it with a system that doesn't bother to support many of the most popular addons.
The sad part is -- I genuinely believe they were trying to do the right thing with that.
Slavishly copying Google's plans to nerf the addon interface isn't "the right thing". Thankfully, Google has gone too far by crippling ad blockers and Firefox is unlikely to fall on that sword.