We don't see any gecko based apps like we see WebKit based apps (electron, etc) because gecko is so tightly developed around Firefox.
I suppose the idea was largely ahead of its time; in 2006 computers just didn't have enough RAM/CPU to get away with running a full web browser stack for every app like we (sort of) can today.
I still think that building a html+javascript package, and have the default browser run it chromeless with some expanded privileges (or near chromeless) is a great approach for cross-platform apps, sadly it doesn't seem like anyone is working seriously on such an approach.
You think so? I find that running a whole second OS for every process is hugely bloated and slow, and that we can do much, much better when there are already very good languages that run cross-platformly, they just need bindings to a good cross-platform UI toolkit.
Unfortunately, nothing looks as good on OS X as Cocoa does.
Look at R studio. Wonderful IDE, runs as a safari app. Having a whole extra browser and a node backend running for each app seems a waste, but using the browser that is already running as a runtime environment seems much saner.
I do most of my work in the browser, I wouldn't mind if some of that had better integrated UI.
I also wouldn't mind writing small, easy to produce apps that will work across systems.
I think the thing that kills that approach is the default browser, which on Windows is IE 12 (Edge).
Edge isn't actually that bad to work with these days. But almost no web developers will use that when they could ship Chromium / Electron just as easily and get the exact same browser on all three platforms.
Even Microsoft's own teams will sometimes make that choice (VS Code using Electron.js and not Edge, for example)
You can see bits and pieces where Microsoft's teams are also experimenting with Node on ChakraCore and Electron on Edge.
If I want to use a web version I can do it using a browser of my choosing not the one developers want.