They're not an multi-billion dollar ad company that can pay to make their browser the de facto default.

In all HN Firefox threads with a sufficient number of comments you’ll see a variation of this comment, and people answering how Firefox fails as a daily driver for them.

Firefox is missing a ton of very useful stuff for some people, such as some audio API or AppleScript¹. The latter absolutely kills it for any kind of automation on macOS. It’s the sole major browser on macOS without support for AppleScript but Mozilla people think that’s just a power user feature. I know because I talked with devs at various points, in person and online, and was given the same answer. Thing is, power users create automations for non-power users. When the latter want to use a tool from the former and the answer is “you can’t, Firefox doesn’t support it”, there’s another user lost.

On a personal anecdotal level, I also don’t find it that stable. Just recently I wanted to try an extension and decided to do so on Firefox. After a while it just removed the extension from the list and it was seemingly impossible to get back, even after reinstalling. Every time I give Firefox a try, I am disappointed.

I’m glad it works for you and many other people, but the answer as to why it isn’t more popular is that your experience is far from universal.

¹ Whose bug report has been open for 22 years: https://bugzilla.mozilla.org/show_bug.cgi?id=125419

For me the deal breaker is that it doesn’t use native UI elements on macOS which means patterns such as invoking menu items using the keyboard behave differently.

They lost a lot of their userbase back in the day when they started getting slower and Chrome was way faster. Now, Chrome is default on Android devices and people are used to it on desktop. Edge is default on Windows, Safari on Mac and iDevices. The only place Firefox is the default is desktop Linux.

Nowadays they've caught up on speed pretty well on desktop, but are still behind on security, and years behind on security on Android.

Personally, I just like features like tab groups, and hate the Foundation's politics, so it was easy to drop it back when they said we need more than deplatforming. The competition has better features, is actually developing new features, and is focused on toolmaking instead of activism. Funnily enough, it shows up in the products.

I used it from when it was Phoenix until 2011 or so.

I left because it was a lot worse than my main two other options on MacOS at the time, and they’d not followed through on the things that made me like it when I adopted it, so I hadn’t really loved it since late 1.x or early 2.x.

I haven’t gone back because there’s no compelling reason to switch back from Safari.

It is still my mod-downloading browser on my Windows gaming PC.

IMO. It's mostly due to not being default browser for major platforms.