How to check if multiprocess e10s option is enabled in your Firefox
Open the site about:support in Firefox, which indicates if e10s is enabled.
There is a line "Multi-process staged rollout", which would be set to true if e10s is enabled. Also look for a number higher than 0 in the "Multiprocess Windows" entry (this is translated to your locale, e.g. "Fenster mit mehreren Prozessen" and "Web-Inhaltsprozesse" in german).
e.g., accessibility, add-ons can trigger disabling this feature.
On this site you can check, if add-ons, you are using are compatible: http://arewee10syet.com/. There is also a mozilla site to check compatibility, you find the addon-id in the extension-filenames in your profile folder, for example the "Tab Groups" add-on has the id tabgroups@quicksaver and is incompatible with e10s.
If you would like to opt-in to test the feature anyway, open about:config and toggle browser.tabs.remote.autostart to true. On your next restart, e10s should be active.
Force Enable
Don't try this on Android, or it will break your Firefox app completely!
On desktops,
if you've tried enabling e10s but about:support indicates that e10s is disabled (e.g., accessibility, add-ons can trigger this), you can force e10s on for testing purposes. Within about:config create a new boolean pref named browser.tabs.remote.force-enable and set it to true. This is not encouraged, use it at your own risk!
(I tried it anyway and it worked, even with Tab Groups Addon still enabled but it's risky!)
Source: https://wiki.mozilla.org/Electrolysis#Force_Enable
My configuration is:
browser.tabs.remote.autostart : true
extensions.e10sBlockedByAddons : false
extensions.e10sBlocksEnabling : true
After these setting my about:support is showing that multiprocess Windows is enabled by user(me).
The first two options
browser.tabs.remote.autostart and extensions.e10sBlockedByAddons is important and third extensions.e10sBlocksEnabling doesn't seems to affect much. One important point is that extensions.e10sBlockedByAddons : false may break some of your addons.