A popular misconception is that Firefox runs Gecko. And while that is kinda true, the real problem is much more interesting when you come down to the technical details.
Because it's the other way around. Firefox doesn't run Gecko, Gecko runs Firefox. Firefox is built in Gecko. In a similar vein, Thunderbird also runs inside Gecko. It's why they look so similar despite one being a browser and the other being an email client. Gecko is, in a way, a proto-Electron.
You cannot "rip off" Gecko from Firefox and embed it inside something like you can do with Blink/Chromium (unless you're on Android and use GeckoView), which means the only way to have a "Firefox based browser" is to fork the entirety of Firefox. There are forks like the TBB or Librewolf that do this, but the embeddability of Chromium makes it much easier for devs to make something that diverges from Chromium in major ways (stuff like Qutebrowser, for example)