Legacy extensions have been disabled now and that also means the way of injecting custom JavaScript into web-pages has changed in a major way. I've reviewed this extension a little more than a year ago when WebExtensions for Firefox were just starting to pop up more frequently. Since then it has become ready for productive use, as none of the issues I encountered last year remain. For that it deserves the full rating.

There's just one little thing to find fault with and that's the advanced editor. You're less inclined to need it as a pure user, but when writing or modifying userscripts, it has been helpful. For some reason it runs buttery smooth on Blink-based browsers such as Chrome or Opera, but use it in Firefox and the extended functionality keeps freezing the tab or textarea for short periods of time. The syntax highlighting is actually fine, but the JSHint-based syntax checking doesn't work nearly as well as it does on Chrome. This is annoying because it distracts from writing code on-the-fly and why I've started using a third-party editor to copy/paste my results over from.

Edit: a comment to another reviewer: the new tab that opens with every extension update is indeed annoying, but this can easily be disabled in the settings (-> Appearance -> Show TM update notification: Disabled -> Save).