nachdenki: YES Works with 3.6! Rated 5 out of 5 stars
I'm no dev. The best I had done before was extract and manually alter max versions in the rdf's before Nightly Tester Tools. However, your code did the trick. I copied and pasted it to a txt file, renamed it chrome.manifest, and zipped it all back up as an xpi and I have fetch text back in the 3.6 FF context menu!
I hope the original dev hasn't left this behind (post grad?). It's one of my faves.