Greasemonkey has release the 0.9.20 beta version that fix the timer problem in version 0.9.19. I personally tested out script, it can run without problem using this version. Below I the link to download the latest version of Greasemonkey. Please note that it is a beta version thus you need to manually down it as Firefox won’t automatically update it for you.
Greasemonkey for Firefox: https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/#beta-channel
Happy Hunting and Happy playing Diablo 3 if you bought one.