5 easy steps to make it work on FF 4.0 Rated 5 out of 5 stars
This is step by step workaround. Thanks to jandys!
1. Download QuickJava .xpi file - you can do this with another browser (not Firefox) - Opera, Chrome, Safari...
2. Extract the .xpi file - if your archive application can't do this, rename it to .zip and then extract
3. Fnd the file "install.rdf", open it with Notepad or such plain text editor. Find the text
(5th row from the end) and rename it to
(this is the highest version, so it could be even higher)
4. Compress it back again
5. Go to Firefox>>Add-ons and load the extension from external file - you will find this option in the top right button (the icon is like a gear-wheel)
That's it. It works perfectly. Tested!