I have this problem from time to time. Usually it seems to be caused by the browser background daemons large memory usage, even when browsers not open (at the moment "browserd -s 5380 -n browserui" takes 15.6% of memory and "browserd -s 5387 -n RTComMessagingServer" another 9.4%).
It seems like the addblock extensions is loaded with the background processes, because the size of the adblock blocklist (patterns.ini, 209kB) seems to directly affect browserd memory usage. If I delete the blocklist files, the "browseui" process only takes 6.3% of memory but "RTComMessagingServer" stays the same.
So I guess I need to look at alternatives to adblock...