I read it as "There's zero appetite [on the part of browser vendors] for introducing non-standard language runtimes into browsers these days".
Anyway, WebAssembly [0][1] is coming through the pipeline with the goal of opening a common browser runtime to more languages. Instead of Mozilla incorporating a Python runtime into Firefox, a Python runtime will eventually be able to target WebAssembly. For Python specifically, the work in progess on builtin GC support should help a lot [2].
As for the donation to PyPy, Mozilla uses Python heavily in internal tooling so I imagine that's part of the motivation.
[0] https://webassembly.github.io/