i have the same opinion parnote
still, 20% of a big dual core cpu is alot, 600mhz more or less for just play a simple video that in a normal program like vlc or smplayer in fullscreen is absurd!
i have experienced the same problems you describe in windows and linux on different machines but also not sure if firefox does work that well on linux, it is like it works in something similar to a windows emulation mode making it ask more resources that on windows
i'm using opera as a plan b, similar results as what you report with konqueror
supposedly firefox 4 changes things but only time will confirm this
"Yet, even still, I don't understand why it would use substantially more cpu in FF compared to Konqueror.
Is Konqueror using something else to play these videos ?"
afik no, if it calls a flash player to show the video, the problem is not the content itself, it is flash player itself that uses too much for this simple task