It wouldn't even be "ending it", really. It'd just be making it the focus of a true FOSS project.

So long as someone still has the source code for the latest build somewhere on the internet, and that code builds into a working executable, Firefox still lives. It's all of the other crap that dies, the crap that no one asked for.

FF needs to be run like the group handling the Linux kernel or the Python Software Foundation. There's a BDFL or a trusted group handling the general direction of the project, and volunteers who give their time to develop the code. Occasionally there are sponsors who throw a few bucks the way of the volunteers so more concrete efforts can occur.