VR is the new sexy thing. Who wouldn't want to contribute the big chunk of VR code to firefox that potential millions of people will be using? The problem is that Firefox has over 40,000 other small, unsexy bugs, including some that are almost 15 years old. There's no corporate management who can say "this stuff is embarrassing, hey you, you gotta fix this before we can even consider a big new feature."
It's not a bad thing necessarily, just different priorities that can potentially result in bloated software. Hopefully "the next big sexy thing" will be streamlining Firefox to make it more efficient, and focus will be directed toward that.