Yes, particularly Firefox is pretty excessive. On FSB30 I was able to run it with 6 windows and about 500 open (but not loaded) tabs in total (needed about 10 seconds to load and maybe 3…6 GB of RAM). On FSB31, starting Firefox with all my tabs blocks my system for several minutes and then is lagging really bad. But also other programs make excessive use of RAM. Chromium for example uses ~800 MB (I just start WhatsChrome, not Chromium itself).
So I now just use Firefox in private mode, and even then my System starts lagging after some time, when everything I have open is Spotify, one Epiphany WebApp, Geary, Firefox with <10 tabs, gedit and Nautilus. This load was not even the slightest problem on FSB30, something seems to be wrong here. 
// Edit:
So with the latest version of FSB from today or yesterday, this has bettered a lot, still not completely resolved, but I guess people are working on it, as the first FSB31b images were even worse. The thing I currently noticed while having an htop window open, is, that going into the overview via pressing Super, allocates enormous amounts of RAM in the range of some gigabytes (more Swap than RAM actually). This then leads to screen (and sometimes even audio) freezes in the range of 10…20 seconds. Selecting other windows via Alt + “the key above tab” sometimes has a similar effect.
// Edit 2:
And the media keys often don’t work and make a restart necessary…