This blog post is a quick explanation of how to setup clang_complete with mozilla. Note: You don’t need to build with clang to use it to complete, this means you can use it for Fennec.
Since these instruction will change from time to time I put them on the wiki page for everyone to help correct errors:

hi, I tried following what you have done here, but the completion is extremelly slow. Is my fault or is normal?
Have a nice day.
By: mattia.valzelli@gmail.com on July 22, 2012
at 1:43 pm