I've never been a fan of the regular release schedule of Firefox (or software in general). Releasing a new version just for the sake of having a new version every three months seems like a way to just make sure you're introducing potential new issues in your software (... lo and behold I think that's what we see with Firefox more and more, rather than the introduction of great new features).
I understand the motivation though - it's nice to have targets to keep everyone working for those little milestones, and have a date attached to it so things can be roadmapped and planned and all that.
I don't think it's at all a big deal for a date to slip on a particular version - especially as we're getting into actual serious-change Firefox territory with this release. The Electrolysis stuff is the first major advancement (... that I've cared about) for something like 20 versions so I'm keen to make sure it's stable.
As an anecdote, the current version of Firefox is the first one that I've EVER noticed it feeling sluggish and like it is using too much memory. I know Firefox has a weird reputation has a memory hog but I have personally NEVER noticed this despite it being my sole browser for years. As of right now it's using 1.9GB whereas before this I don't recall it getting significantly above the low 1GB range (FWIW I have Electrolosys disabled by config).
I don't really care that much about the memory usage but it certainly feels a little more sluggish than usual, which I do care about. So I'm very happy for them to take their time with the v49 release and make sure it's all ship-shape before it lands.