Tampermonkey is the most popular userscript manager, with over 10 million weekly users. It's available for Chrome, Microsoft Edge, Safari, 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 (Google Drive, Dropbox, OneDrive), automatic update checks and also browser and cloud storage based synchronization. And finally it's compatible to Greasemonkey up to version 3.x with experimental 4.x support.
Note: Firefox's WebExtension API is currently considered to be stable, but not complete. It still has incompatibilities and some issues. This means Tampermonkey has (besides it's own bugs ;) still some issues caused by faulty, non-existent or incompatible APIs and/or features.
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 (Google Drive, Dropbox, OneDrive), automatic update checks and also browser and cloud storage based synchronization. And finally it's compatible to Greasemonkey up to version 3.x with experimental 4.x support.
Note: Firefox's WebExtension API is currently considered to be stable, but not complete. It still has incompatibilities and some issues. This means Tampermonkey has (besides it's own bugs ;) still some issues caused by faulty, non-existent or incompatible APIs and/or features.