Okay, let's be honest here. I'm a web dev for around 10 years and I really, really used to love Firefox. But, I'm being honest. Firefox is installed on my machines only for one-time website tests, that's it. And IE9 in a VM is too.
Reasons:
Really, the only thing I value in Firefox is that it is using not system-specific proxies. That's why it's basically my hyperboria and TOR Browser. But I'm honest here as well, I simply build my own GNOME shell extension to switch Proxies via one-click, so I can't value that as a real argument.
In my personal opinion, Mozilla realized that above points are a problem. Heck, they created the most awesome paralleled programming language for that.
BUT in order for Mozilla to stay alive, I think they have to push Servo further. The simple "step-by-step" migration plan didn't work for the last 5 years, so it won't work for the upcoming 5 ones. I mean, every single damn update in Firefox makes more confusion about how to create an extension.
It simply got impossible to google. Any extension on github that is older than a year (or was a tutorial) is so out of date that it even won't compile anymore. So that's my argument about jetpack, the most idiotic undocumented and stub API ever created. Really, any tab-injecting API is a stub API and somewhere in the issue tracker as on hold. WTF.
Letting my personal opinions on those matters out of place, I value the spirit and goals of Mozilla. But they really have to change the course, otherwise they will lose people like me forever. The new ad-platform heavily reminds me of sourceforge; and everybody knows how people think about sourceforge now.
TL;DR
Get your shit together, Mozilla. Give us a Browser based on Servo and Rust, make the extension API compatible with the WebApps WG-based API of Chrome, nw.js, Opera, electron (and pretty much the rest of the extension-relevant world) and everybody is happy. Don't reinvent shit from scratch if you don't have to; contribute.