Hello, first of all, keep in mind that if you tried it on addon.mozilla.org, Mozilla doesn't let any extension run in this website unfortunately.

With that in mind, try this:
1. Make sure you are running the latest Firefox version (57 or newer)
2. Try to enable on a website with obvious grey text, like goldbrecht-systems.com
3. If all else fails, try the new version 1.2. It's currently in the beta channel but I will release it soon.