It's interesting, often prompted by Mozilla doing something particularly silly I try out the competition and every time I go back to Firefox utterly unimpressed by the supposedly better Chromium based browsers out there. Honestly I don't see what people like so much about them.
Most recently when they came out with Colorways I decided to give Vivaldi a shot for the first time in a few years, since it has that easy custom color theming without a pointless time horizon. First problem I ran into was that the built-in ad blocker breaks YouTube. Not a great first impression, but hey you can just disable it and install uBO. But I quickly came to miss the flexibility of Firefox's interface. On the surface Vivaldi is very customizable, but you quickly run into a wall when wanting to go outside what they've built. For example, you can put the tab bar anywhere, but you can't have it in multiple places or pretty it up beyond changing the colors. Firefox on the other hand has enough tab management addons for any taste, plus it supports custom CSS within the addons themselves and at the browser level.
The alternative browser I've been most impressed with is actually Edge, but I can't tolerate it constantly shoving features I don't want in my face or the mandatory telemetry.
So to answer the question, I would save Firefox by breaking the mobile browser duopoly. Desktop Firefox is already obviously better than Chrome and Edge, even in the basic experience with no addons, but people just use Chrome for some reason. I think it comes down to habit, an over-reliance on Google Apps that work better with Chrome's tight integration, and familiarity due to Chrome being the only serious browser on Android. And on iOS the situation seems to be even worse: non-Safari browsers are forced to use Safari's engine anyway, and all of them offer a noticeably worse UX than Safari so why bother. Of Firefox's problems, losing on mobile is the easiest to fix, not that it's super easy. Mozilla "just" needs to focus creative resources on building a compelling alternative browser on Android and a functional one on iOS. That would go a long way toward bringing users back.
Another thing that might help is for Mozilla to make a clear (down to earth, jargon-free) statement of its values and goals as a nonprofit. I think a lot of the criticism Firefox gets in tech circles isn't exactly sincere, because many people have switched away from Firefox due to actual or perceived political differences but don't want to come out and say that, so they contrive or exaggerate some UX or privacy issue. If Mozilla's leadership would speak openly about these issues it might make those detractors a little more comfortable saying something like "I don't use Firefox because the causes they support go against my political convictions", rather than the current situation where they might be reluctant to say that and start a likely pointless argument over whether Mozilla supports a certain cause or not.