Tampermonkey is the most popular userscript manager, with over 10 million users. It's available for Safari, Chrome, Microsoft Edge, Opera Next and Firefox.
Tampermonkey makes it very easy to manage your userscripts and provides features like a clear overview over the running scripts, a built-in editor, ZIP-based import and export, automatic update checks and browser and cloud storage based synchronization.
This extension is installed in just a minute, so give it a try!
Note: Please use Tampermonkey at macOS 10.14.4+ for optimal results.