Installing User Scripts
How mature is Tampermonkey for Firefox? I think from the description that it should be treated as somewhat experimental.
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 or non-existent APIs and/or features.
Installing User Scripts
I recently check out this page and I missed the listing for Tampermonkey for FireFox and MS Edge. It would be great if they were added as well.