WebAssembly started at Mozilla, but is very much not controlled by Mozilla today. It's a W3C Community group: https://webassembly.org/community/contributing/

That should be the goal of any browser vendor: collaboration towards getting new tech standardized.

Is it really ? I was under the impression that a large influence and a precursor to wasm was NaCL from Chrome along with ASM.js from Mozilla. Also Microsoft seems to be very involved in WASM.

If they're not all involved, it won't work, but firefox really lead asm.js, which was the actual precursor to webassembly. NaCL was more like a competitor that tried to do something similar but never got the kind of mainstream adoption needed for cross-browser support.

NaCL is more like Google's ActiveX. wasm directly builds upon the experience and success Mozilla had with asm.js.