Rated 5 out of 5 stars

Oracle has changed the search URL. Locate and open the file "mysql-documentation.xml" within your Mozilla's searchengines-directory.

Then change 2 lines by updating the search URL like this:
<SearchForm>http://208.49.150.34</SearchForm>
<os:Url type="text/html" method="GET" template="http://208.49.150.34/search?q={searchTerms}&amp;site=documentation">

Now the search addon works fine again!

This review is for a previous version of the add-on (20090413). 

Please use the new version. Oracle moved the search URL to a new location. It's now up to date in version 20110126.