Tampermonkey is now available in Firefox Nightly, and here's a place to discuss it.
I currently do not have Nightly installed on my device, but I do have Beta, and added TamperMonkey to my personal collection after the announcement. I've found a good number of helpful userscripts, but I feel the experience of editing them and importing them could be better. The text-box for text editing is sluggish, and the browser seems to freeze when importing the scripts I found and edited on desktop.
I can only hope the experience is currently better on Nightly, because that means not only will it eventually be better on Beta, but it means there's hope of getting this into stable.