Firefox gained all the popularity in the "primitive ages" when it was the first one to offer a decent ad-blocker.

It basically spread when tech-savvy users recommended it to others ("Don't want to see those popups? Get Firefox with ad-block add-on") or straight out installed it to their relatives, because they didnt want their granny to click those ads.

Mozilla foundation works very hard to shake off this "power user" group (people who want and can customize their browser), while pursuing some undeclared group of other users. Those other users never materialized.
They probably use Chrome, because it "just works" and doesn't do the batshit insane things that Firefox tea, did repetitively.

Here is just a short list of things that Firefox did to shake off their core group (people who want to customize the browser):

* requiring a signature for add-ons, what killed many of them (many niche were not signed, since their authors didn't care about some small add on with few hundred users)

* One year later after requiring every add-on to be signed, it turned out that MOZILLA FOUNDATION forgot to renew their signature, so all add-ons stopped working... the incompetence here is incredible

* Then they "changed the technology" what meant killing all the existing add-ons. Lots of the popular ones could not be rewritten, since the new (and supposedly better /s) Firefox does not have proper APIs. Why? Because writing those APIs is hard and the Mozilla developers didn't want to do it. Please note: it is not that they work for free, they earn 200-300k USD per year, get 12month golden parachutes when fired and whole foundation burned through hundreds of millions of dollars. So dont say they didnt have the money to write a customizable browser. They just didnt want to do it (in fact we could argue that they already had it with Firefox up to version 56, but bug fixing it is not sexy; while writing new stuff that nobody wants is a great boost for your CV)

* adding things that users dont want or need - e.g. updates via telemetry, despite supposedly being the "not spying" browser

Now Firefox is basically a Chrome clone that claims to offer privacy, which it doesn't (telemetry). Also if you use it on mobile, it will kill your (last few) add-ons again. I think at some point it only allowed 7 add-ons?

Why use unstable and unpredictable turd? Chrome is disappointing (due to lack of customization, messing with address bar in order to promote AMP...), but at least predictable. So users switched. With Firefox you don't know what they will break next.

Bonus comment: go and explain to your grandmother that "new and better" Firefox team does not allow you to customize UI anymore, since API that allows to select options on right click does not exist anymore, because 300k USD per year programmers don't know how to write it.
Yes, there was that add-on to customize right click (90% of options are not needed for anyone) - and there is no such add-on in new Fireox. Classic Theme for Firefox - which made it look the same (so predictable for old people) -> impossible as well.
We are in a thread that some UI option is killed just because it gets killed.

And I talk here about random users. Power users had a lot of more niche add-ons, which did what they wanted. All those add-ons are lost now. There were thousands of them.