While it was originally scheduled for last week, some last-minute issues preventing the software non-profit organization from releasing it until today. Also, for some reason, Firefox for Android doesn't want to update from within itself, but triggering an update from the Google Play store works. This might be temporary and/or happens with every Firefox for Android update; I'm new to this platform.
With Firefox 49, VentureBeat is reporting that Mozilla is allowing a “small initial set of compatible add-ons” to be enabled alongside multi-process. If you don't have any non-compatible add-ons installed, then you should see Multiprocess Windows enabled in about:support. Otherwise, it will be disabled and you won't see any difference.
Interestingly, Mozilla is promoting "Refresh Firefox" at their site if you have the latest version. This basically cleans all the add-ons out of your user profile, but maintains browsing history, bookmarks, and the like. It might have been around for a while, but, if it's new, it times nicely with the multi-process rollout. On top of cleaning out old, crufty add-ons, a user should see a bigger jump when Mozilla's enhancements are (I'm guessing) enabled.
Mozilla has also changed a few things here and there, too. While many of our readers will probably have hardware acceleration for video, they have just added in SSSE3 enhancements if GPU support isn't available. I'm not sure all of the use cases for this, but I'd expect it would help in virtualized environments and certain, older PCs (ex: Intel Atom and Via Nano). I'm just speculating, though.