Who knew that continually ignoring your userbase and changing things in ways they don't like for roughly a decade could have negative consequences?
The list is getting too long for me to even remember, but I'll try: they moved tabs to the top (can't even toggle it via about:config anymore), they killed regular download dialogs, they killed the regular status bar, they removed the ability to keep browser history but not keep download history, they radically changed their address bar search function (Awesomebar) and appearance and provided no option to use the old method people were used to, they made accepting a self-signed certificate more difficult than filing your taxes, they fought Debian over petty license branding issues (that other software had no issue with) giving many of us "iceweasel", they radically altered their interface to be a poor Chrome clone and killed all customization (can't put refresh button on the left, can't unmerge back/next buttons, etc), they started putting adware onto their new tab page, they made it so that extensions must be signed by Mozilla to be installed with no ability to override, they turned HTTP/2 into an agenda by making TLS mandatory in spite of the IETF's decision on that. They continue to blow off per-tab process support, and 64-bit Windows builds are still not mainstream. And that's off the top of my head, I'm sure there's more. Eich doesn't even have to factor into this, no matter which side of that you're on.
You can like or hate any one of those, and yes if you want 20 extensions you can mostly make it look and act like it used to. (Plus, they talk about removing all that stuff to simplify and unbloat the UI, and then they add useless crap like Firefox Hello in its place.) But each time they changed things and completely ignored their user's feedback, they lost a few more users to Chrome. I don't really like Chrome all that much either, but at least it's not a constantly changing target, where you never know what feature you're going to lose because of an auto-update.
Firefox's decline wasn't any one great catastrophe: it's been death by a thousand papercuts.
It's really simple: if you offer a feature at one point, and you want to keep your users happy, then you don't completely remove that feature from them in the future. You can default to something else, fine, but you make an effort for people who liked the old way. Microsoft understood this up until Windows 8. And it looks like they're relearning that lesson again a bit with Windows 10's changes.