Still works in FF11 Rated 4 out of 5 stars
I can confirm that yk2000's instructions still work in FF 11. (See comments below.) At first, I got the same error as NottyWizard, but then I realized that my ZIP program changed the directory structure. Compare it to the original and make sure they are the same.
You also need the "DOM Inspector" addon to make this work. See the screenshots for help getting started with Xpather.
I only gave it 4 stars because it was such a hassle to install.