Just what I was looking for Rated 4 out of 5 stars

I've been using the Translate extension for a long while, but this extension has the two features I've always wanted.
* Can configure for one-click translation to a favoured language pair (I mainly translate Dutch to English).
* Adds extra translation services (Translate only uses Babelfish.)

Only downside was that although Google translation supports Dutch-English, Translator doesn't know about it.

As I have managed to work out how to add this functionality, here's a summary for anybody who needs to do the same with this or any other language pair. (I did it on Linux, it should be similar on other systems)

cd ~/.mozilla/firefox/*.default/extensions/translator@dontfollowme.net/chrome
jar -xvf translator.jar # Extracts the contents of the JAR file
cd ~/.mozilla/firefox/*.default/extensions/translator@dontfollowme.net/chrome/content
vi translator.properties # (or use your favourite text editor)
Find the line starting "google.langpairs" and add in the relevant codes (for example, to get Dutch/English and the reverse, I added "en.nl,nl.en")
The next step may or may not be needed, I needed it to get it working with my en_GB locale.
cd ~/.mozilla/firefox/*.default/extensions/translator@dontfollowme.net/chrome/locale
cp -r en-US en-GB
cd en-GB
vi translator.properties # (or use your favourite text editor)
Add lines for your language pairs in the same format as all the other lines.
cd ~/.mozilla/firefox/*.default/extensions/translator@dontfollowme.net/chrome
jar -cvf translator.jar * # re-builds the JAR file

Re-start Firefox and you should now have your new language pair. If the pair doesn't appear for your chosen translator, check to see if Firefox has output a message like "*** Failed to get string google.en.nl in bundle: chrome://translator/locale/translator.properties" if it did, then you need to do an edit on locales similar to the one I did.

Once it's working, you can delete everything in ~/.mozilla/firefox/*.default/extensions/translator@dontfollowme.net/chrome apart from translator.jar

This review is for a previous version of the add-on (