Firefox52esr works without problem. Reproducible: always Steps To Reproduce: 1. Make sure e10s is disabled 2. Install e10s incompatible legacy addon such as "DOM Inspector" Actual Results: The addon is automatically disabled. No Restart now button display. Expected Results: A Restart now button should display.
You can work around this by flipping the preference extensions.allow-non-mpc-extensions. Given that this only affects Nightly and it will change significantly in 57, this isn't a high priority.
Neither non-e10s Firefox nor non-multiprocessCompatible extensions are things we can spend any time on supporting.