I just noticed that Firefox froze. 'top' told me that process plugin-container ate all the CPU. 'ps -Ao args|grep "plugin-container"' returned:
/usr/lib/firefox-21.0/plugin-container /usr/lib/mozilla/plugins/libflashplayer.so -greomni /usr/lib/firefox-21.0/omni.ja -appomni
So in Firefox' menu I found Tools => Add-ons => Plugins and disabled Shockwave Flash.
PS of course other plugins could be involved, so you'll have to check which one is in each case.
EDIT. And yes, I know that "flash is bad", but that's not the point here.