Because it's one of the only remaining browsers (the other one being Safari) that doesn't run on Chromium. We must protect FireFox and Safari with our lives because if they die out then Google has a monopoly on the browser space. Not something anyone wants... I mean look at their Manifest V3, and web DRM controversies. They are trying to ruin the web. Don't let them people!
Plus, I just like the ability to customize the toolbar, and FireFox Sync is just brilliant for syncing between mobile and desktop!
Hmm, I bet most of the functionality could be replicated using a browser extension. Pretty much the only thing I think you wouldn't be able to access would be saved passwords and credit cards. Networking might be an issue as well if you were trying to set up an ad-hoc system like KDE connect uses.
Can't have Google kill Firefox, if Firefox does it first. Every few years they (IMO) fuck something up that makes me want to move away, but there's no real alternative :/