* They've alienated many Firefox users thanks to many bad UI changes. They've continued to do this even after the users have strenuously objected to these unwanted changes. Firefox's share of the market has thus dropped from 35% to probably sub-10% these days.
* Many of the remaining Firefox users still point out that Firefox is slower and more bloated than other browsers. Although Mozilla often rejects or ignores these complaints, my years of software development experience have taught me that when many users say there's a problem, there very likely is one, even if we the developers can't reproduce it.
* Firefox for Android hasn't been picked up by many users.
* Firefox isn't an option on iOS. (Although I guess we can't fully blame Mozilla for this.)
* Firefox OS is floundering. The devices available so far have fared very poorly in reviews. Some of these reviews are among the harshest I've ever seen for any software or hardware product.
* Thunderbird is on life support.
* Rust is still pre-1.0, and will be like this for several more months, at the very least.
* Servo depends on Rust, so it being a viable option is still years away.
* There was that whole Eich debacle. It was pathetic, no matter how you look at it.
* Bugzilla is long forgotten these days.
* Despite absolutely massive funding from Google and now Yahoo, Mozilla hasn't managed to put out any other product that people actually want to use.
When I look at that track record, it's just one failure or disaster after another. It's not top-notch at all. So I'm not surprised that they have trouble using C and C++. They seem to be having severe trouble with pretty much everything they do!