I recently stumbled upon the Firefox search engine add-on "Qt" which lets you search doc.trolltech.com. I gave it a try... it's broken. Good thing is fixing it was very easy. Here is a fixed version for download plus a direct activation link:

Add Qt search engine

To view the installed description file (version 7.2, XML) click here. The original engine (version 7.1) by Daniil Ivanov can be found here.