Per-site process isolation is a powerful security feature that seeks to limit exposure of a malicious website/script abusing a security vulnerability. Firefox calls per-site process isolation Fission and is enabled by default on desktop. Fission is not yet enabled by default on Android, and when manually enabled it results in a severely degraded/broken experience. Furthermore Firefox on Android does not take advantage of Android’s isolatedProcess flag for completely sandboxing application services.

Obviously Firefox has it own data isolation, but this doesn’t matter if someone can execute bad actiing code due to lack of process isolation.