Big part of the lag isn't due to flash itself or firefox.They actually do a huge work.
Sadly flash plugin is often shared with ads in sites and flash is used in almost every ad because it's easier to develop than javascript.
Chrome instead open a new process for every tab,so there is a new istance of flash plugin for every tab and flash plugins in inactive tabs are paused and restored when you switch on or remains active if cpu allows it.This assures that every tabs perform at the best, but uses up more RAM than firefox.
Firefox can do the same thing if you open more windows instead of more tabs.
IE lately has caught up the gap with chrome and firefox and there are milliseconds of difference about them.IE can be also faster than both as it has native support (i can say that the whole os is built around it!) and most sites give it special attention since is the most widespread browser.