[Updated] Greasemonkey version 0.9.20 beta released, check out my latest post
What you can do then? For the mean time, use other browsers like Opera or Chrome until there is another update by Greasemonkey that fix this problem. Or you still insist want to use Firefox? Then you will need to downgrade your Greasemonkey version. How to do it? First, uninstall your Greasemonkey add-on in Firefox then restart your Firefox. Then go to install version 0.9.18 instead. you might need to reinstall the user script too. Please note that your friendly Firefox will try to help you update the Greasemonkey to the latest version if you restart Firefox.
Let’s keep our fingers cross that Greasemonkey release an update to fix this mess. I will update you all when a better solution found.