Wired is pointing out that Firefox is now "flatlining", after dropping in browser market share from 30% in 2008 to less than 4% today. They're 100% right: Firefox has no clear future.
I'm a niche user, so while I have strongly-held beliefs about what I like in software, I know that most others won't care about those same things. But since Firefox is bleeding users at high speed, I'm going to outline what I would like to see in Firefox that could give people a reason to use it again.
Split View
Remember browsers before they had tabs? Opera started the craze, and it took off and was adopted across all browsers over a few years. The next frontier is a split window view. While I normally would argue that the window manager should be doing this, most folks don't have mastery of their window manager, but could easily make use of a "split vertically" and "split horizontally" option.
Better Bookmarking and History
Lots of folks leave tabs open forever, and when they try to use bookmarks instead, they find themselves overwhelmed with bookmarks. This lack of organization within the browser opens them up to having companies organize their information instead: find that Twitter post using Twitter instead of the browser, just search Google again to find that recipe you were reading yesterday, etc. As a privacy-centric product, one thing Firefox can do is have a UI that pops up when the user is tying in the omnibox that progressively filters all their bookmarks based on the input, prominently highlighting when the bookmark was created and the last time it was visited. This is sort of available using * <bookmark name>, but so few know about changing search bar results on the fly that the feature might as well not exist for 99% of users.
Keyboard Customization
Many apps allow keyboard shortcuts to be customized, but browsers tend not to. In particular, it seems silly that extensions can't overwrite defaults like Ctrl-w, Ctrl-p, Ctrl-n, and Ctrl-t.
Serious Tools for Addon Management
Every addon is now a potential supply-chain attack against end users, so there is a lot of value in vetting high-profile extensions. They currently do this for extensions like uBlock Origin and Singlefile, but not other heavyweights like Vimium, Dark Reader, HTTPS Everywhere, and Decentraleyes. Providing a system that not only vets the code during install, but also when updates happen has a lot of value to folks that want to customize Firefox's behavior.
The general theme is that Firefox can give users a reason to come back by offering what other browser can not or will not. Instead, Firefox has been chasing what other browsers do, and this means they are always behind. Rather than focusing on "personalization" that allows users to change the color of the browser chrome, they need to focus on "functional personalization" that allows users to change the behavior of the browser in a safe way.