I got it working in Firefox with the following steps:
1. Install Greasemonkey.
2. Click on the Greasemonkey icon to enable it.
3. Click "install" on the userscript page.
4. Greasemonkey will produce a dialog asking you to confirm the installation. Click Install on this dialog.
(If Greasemonkey does not produce a dialog, click on the Greasemonkey icon again.)
Tatoeba is a database of sentences in different languages; it's not a site for language learning in the usual sense. However, it's very good as a companion to another course to help build on your current knowledge. I hope you'll find Tatoeba useful!
I was just thinking about this earlier. It would be nice to have languages that I've already used moved to the top of the list so I don't have to fish through it. Auto detect helps a lot, but it's not 100% reliable for languages with only a few sentences.