Loosing the general audience was do to Google using their dominant search position to shove constant ads like: "Use a secure, fast Browser: Switch to Goole Chrome!" during user searches.
While browser projects like servo or ladybird are certainly appreciated, I think their state of "something browsable" is not really what most users, even technical ones, would like for their daily browsing.
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How do you think Firefox has lost the power users/devs?
Google didn't steal many Firefox users initially. They absorbed more new users in a rapidly growing market. They used to pay sites and downloads to install Chrome alongside whatever the user actually wanted to install (sometimes done without the user consent)
Later the network effects started going against Firefox but most people go by percentage of market and assume they were losing users long before they actually did.
Not to forget the magnificent growth of Android with Chrome being the standard browser since 2012, with absolutely no incentive to install any other browser.
It sounds like they were just unlucky, but they are part of the loss.
First they loss the mass, that's a Google marketing problem.
But then they loss the power users and that was their issue. (Since then I'm a blink engine user)
If I must rally behind something that is something browsable I would pick servo or ladybird.