Firefox isn't as stable in a few areas. Since it's been released, the main UI has changed drastically many times compared to Chrome which has been the same forever. Not the settings UI, but the main UI and menu system... Furthermore, nobody fucks with Chrome's settings on any Linux distro like they do with Firefox. And then the integration occasionally resets settings and other bad things happen when packages get updated. Can't stand that.

The other kind of instability with Firefox surrounds the corporation that produces it. Mozilla is a bit of a wildcard because you never know what sort of guerrilla marketing advertisement or third party integration they're going to throw into your browser without giving you much of a choice. I actually trust them less than Google because there are way fewer eyes on them.

Finally, in my opinion, it's just not as consistently good as Chrome. I don't think Mozilla can compete with the level of production quality that Google has put into Chrome. Not even close. Where Chrome is modern, crisp, clean and beautiful looking with a great UX, Firefox is just ugly, primitive, old smelling and annoying to use.

I know Microsoft can do better than Moz on the production quality front. Who knows what if their UX will remain stable though. They kinda ruin everything with the constant push for signing up to a Microsoft account. Mostly I care about the production quality and the general UX though.