[Unsupported] Leech training script

You might want to try downloading “Dashboard Userscript: Leech, Apprentice, and Guru detail (aka SRS level progress)” which you can download here:

It’s made by the same guy and basically works off of that in some ways. Might need that script before you can work this one.

That, or maybe something is wrong with your particular browser? Firefox recently changed somethings which might cause a problem with previously downloaded scripts.