Firefox Nightly disables e10s whenever accessibility is detected, this includes A11y tools like remote desktop, touch screen Windows devices and other magnifying or screen capture apps when running on the desktop.

For instance, when you try to capture an area in Nightly with Snagit, with E10s enabled, a notification will be shown that ‘accessibility support is partially disabled due to compatibility issues with the new Nightly features’, if you click OK or choose ‘Enable’ (which requires restart), the multi-process feature will be disabled.

Nightly accessibility support partially disabled

When you visit Startup under Options, you’ll notice this: Enable multi-process Nigthly (disabled: An accessibility tool is or was active), you can get around this and temporarily turn on e10s by changing
browser.tabs.remote.autostart preference value to true.

But, when you use any accessibility tool again, e10s will be disabled, to permanently enable e10s in Nightly when accessibility tool is active, you need to follow the instructions below.

1. Visit about: config

2. Create a new boolean preference named browser.tabs.remote.force-enable and set its value as true, and change  accessibility.force_disabled preference value to 1, from 0.

Now read: Multi-Process Firefox (e10s) Release Date Pushed to 2016