One of the most essential and best addons out there. But, the newest developer version suddenly stopped working in 56.0. I don't know why, it just suddenly disappeared. It might be really buggy at the moment but this never happened before. It doesn't show up as an icon in the menu, either. I've tried disabling/reenabling and resinstalling the addon. uBlock could definitely use an update.

Edit: Restarted firefox completely and it came back. Nonetheless confirms that it can crash itself within the browser and requires a browser restart. It's buggy, but at least it works great. Cheers to the guy who made this and here's to hoping they update it to fit in with 56.0.