Who said otherwise? 
FWIW: We did experiment with enabling isolatedProcess, but it unfortunately proved to be too unstable in our testing… 
I’ll repeat: Who said otherwise? Did I miss a reply somewhere? 
FYI Mozilla has began officially rolling out Fission to Nightly users. It seems to be far more stable than it was in the past based on our research and testing. We also expose a setting in the UI (as explained in the release notes) to disable Fission, so that if it does cause issues, users can act accordingly (Though so far, we haven’t heard any complaints).
I won’t lie that I’m a bit surprised by all the negativity here. Can’t we just appreciate what is objectively a positive step towards improving Firefox’s security on Android? 