How much of the work that went into deprecating certs and removing obscure command line flags and setting up HTTP/2 (which isn't even really used by anyone yet) could have went into electrolysis?
Another thing to keep in mind is that patch notes are a list of user-facing changes, not a breakdown of where the engineering effort went in a particular 5 week release cycle. Removing the command line flag probably took an engineer an hour or two to do. In the same time frame, I spent weeks analyzing and fixing a variety of leaks that only show up in multiprocess Firefox, but none of that shows up in the patch notes.
I agree with you on older bugs just lingering around. This is a serious issue for many open source projects. JWZ was complaining about it years ago[2]. My favourite example is the problem of putting the tabs at the bottom of the window in gnome-terminal[3], which has been reported over 12 ago, has a patch available posted in the bugzilla, and it's still not fixed.
[1] https://addons.mozilla.org/en-US/firefox/addon/spdy-indicato...