There are two somewhat more aggressive things in this version. The first is to even more heavily throttle inactive or background tabs to reduce their impact; the normal Firefox runs animation frames on background tabs at 1Hz but this version reduces that to a third (i.e., a tick every three seconds instead of one). It's possible to make this even more aggressive, including just not ticking any timers in background tabs at at all, but I'm a little nervous about halting it entirely. Neither approach affects audio playing in inactive or background tabs, which I tested thoroughly in Amazon Music and YouTube. This should make having multiple tabs open and loaded a bit less heavyweight, particularly on single processor Macs.
The second has to do with session store. Currently, every 25 seconds (Firefox has a default of 15), the browser serializes its state and writes it out to disk so you can pick up where you left off in the event of a crash. I'm loathe to completely halt this or make the interval much more than 60 seconds, but I also know that this does drag on the browser and can also spin SSD write cycles. This version increases it to 30 seconds, but also reduces the number of forward pages written (up to 5 instead of unlimited), and also purges old closed tabs more aggressively -- Firefox purges these after about two weeks, but we now purge old tabs every 24 hours, which I thought was a good compromise. This means much less data is written and much less cruft accumulates as you browse, reducing the browser's overhead and memory usage over longer uptimes. However, I know this may upset those of you who have tab closure regret, so advise if this drives you nuts even though I am unlikely to reverse this change completely.
This version is timed to come out with Firefox ESR 52.6.0, which is not scheduled for release until January 23, so don't panic if you don't see much commit activity in Github for awhile. 52.8.0, scheduled for May 7, will be the transition point to 60ESR. More on that when we get there.
For FPR6, I'm looking at a couple features to get us more HTML5 support points, and possibly something like date-time input controls or <details> and <summary> support. This may also be the first release with built-in adblock, though the adblock support will only be basic, will not be comprehensive, and may include blocking certain tracking scripts as well as image ads. It won't be enabled by default.
Party safe on New Years'.