I have a six megabit connection and a 1,2 Ghz Centrino Laptop running Ubuntu 7.04. If I download only one file with only one chunk Firefox uses always at least 50 - 60% cpu and with two downloads with one chunk 80 - 90%. I think this is too much and it prevents cpu from scaling down so laptop makes much more noises. Wget btw. uses only around 1%. I know that a GUI needs more but I have disabled the great new tooltip so the difference shouldn't be so great. This is not only a problem of 1.0 beta since the old version has a similar problem but I think cpu usage has increased.
Btw. except of the high cpu usage DownThemAll is a great download manager and since 1.0b it works without problems under Linux and it is the only download manager for Linux I have found which could manage file name changes after download start (filenames different from url).