I don't know... Do they have greasemonkey or something ~the same for Opera?

I looked around and apparently it's a native functionality: http://www.ghacks.net/2008/08/10/greasemonkey-i...
That should do the trick. Tell me if it still works right.

Depends on what part you want to test. You can either poke around with it off and find TGPs that are encrypted and then see if it works on them, or you can write your own page and encrypt the links in the different ways and see if it decrypts them properly.

The first site that comes to mind is http://jizzle.com (Not Safe for Work) which has the plaintext encoding. So if you visit it without this script running it should give you links like http://jizzle.com/out.php?link=blah. When the script is running, you should just have links like htp://blah (or more accurately: http://jizz.jizzle.com/video#####/blah)

If you want to write your own test page, all you need to know is how to write a barebones HTML page and make links.
aHR0cDovL3d3dy5nb29nbGUuY29t should translate to http://www.google.com
nUE0pQbiY3q3ql5ao29aoTHhL29g should translate to the above as well
http://www.google.com/out.php?l=http://www.yaho... should be translated to yahoo instead of google.