[[If anyone finds this helpful or useful please say so I'll continue to keep an eye out and continue this vein, otherwise I'll assume everyone follows the same news sources I do.]]
mac x86/x86-64 has gone away, and it is now only x86-64: https://groups.google.com/forum/#!topic/mozilla.dev.platform/XwUrL4d9Rf0
Fennec now has Chromecast/casting stuff on by default. I'd suggest this should be disabled but that it should also be investigated at some point to see if it's doesn't enable proxy bypasses and might be able to be enabled at the lowest slider level. https://groups.google.com/forum/#!topic/mozilla.dev.platform/Q-dSrp4PYe4
Eventually coming will be OpenType Variation Fonts: https://groups.google.com/forum/#!topic/mozilla.dev.platform/_FacI6Aw2BQ I don't know anything about these specifically, although I'm sure there's font fingerprinting concerns and general font security concerns.
MediaError:message attribute is being turned on by default: https://groups.google.com/forum/#!topic/mozilla.dev.platform/CKTzq7rF50o This looks like something that could be used to fingerprint users!
Installing multiple XPIs simultaneously may be going away: https://groups.google.com/forum/#!topic/mozilla.dev.platform/om5BrXM-Gb8 I don't think this should affect TBB...
Eventually planning on writing some Private Browsing mode data to disk: https://groups.google.com/forum/#!topic/mozilla.dev.platform/Js-pI-6dSQA
There's a NetworkInformation API that got some discussion: https://groups.google.com/forum/#!topic/mozilla.dev.platform/lCZmhCDGHPY It's basically as horrible as one would expect, but fortunately several people raised concerns. Still is important to keep an eye on as it seems eventually it will need to be turned off or hardcoded.
GCC 4.9 is now required to build Linux/Android: https://groups.google.com/forum/#!topic/mozilla.dev.platform/QgxoSzTcp1M
clang-cl has become a tier 2 compiler: https://groups.google.com/forum/#!topic/mozilla.dev.platform/JzHt3ykIfHg I hope to get mingw to tier-2 status also.
There's this new (or maybe old?) thing called compact themes: https://bugzilla.mozilla.org/show_bug.cgi?id=1314091
There are a few features that are brand-new but not known to be dangerous. It's just 'new code'. I'm not sure if there's any appetite to disable these sorts of features at the medium or high security level for a time before enabling them. CSS Grid: https://groups.google.com/forum/#!topic/mozilla.dev.platform/6shk3TZX5vo CSS Mask Image: https://groups.google.com/forum/#!topic/mozilla.dev.platform/15nssyOe1gc Selection::setBaseAndExtent() https://groups.google.com/forum/#!topic/mozilla.dev.platform/YdFwg3VpGMg (no pref for this) html5 dialog element: https://groups.google.com/forum/#!topic/mozilla.dev.platform/vTPGW1aJq24 CSS display:flow-root https://groups.google.com/forum/#!topic/mozilla.dev.platform/bG9Kpgr5LC4 CSS caret-color: https://groups.google.com/forum/#!topic/mozilla.dev.platform/lOpCwCMIWwo (no pref)