This is how Free Software projects work; people that want a feature implement it. It's great that you want something and it's possible that your desires will line up with those of someone who can easily implement them ("oh yeah, great idea, i want line numbers too"), but more often than not, the people with experience on some project are more interested in some deeper problem. If you're hacking on the JS JIT, line numbers just aren't important to you; you never "view source" in the browsers and may not even run the browser all that often. It's likely that your "extracurricular hacking" will be on something like a better test harness or better Emacs integration with your development workflow.
It is hard to go from being a user of software to a developer of that software, especially in this day and age of easily-downloadable TV shows, movies, and /b/ memes. That's why it takes 7.5 years to find someone interested in hacking on this feature, them getting themselves up to speed on Mozilla, and then finally implementing an acceptable patch. It would be nice if someone got paid to work on this sort of stuff, but users are comparing Firefox to its competition with things like Javascript benchmarks and WebGL conformance, not whether the browser has line numbers in view source. That's not to say they're not important, but rather very easy to deprioritize when the people that know how to implement that quickly also know how to implement "bigger wins" more quickly.
Ultimately, a free software project lacks feature X because you haven't added it yet. Remember that when you submit complainy titles to HN. Don't rely on someone else to make your life better because you're going to be pretty disappointed when you realize that you don't matter much to the world at large. Everyone else has other things to do too.