I also notice this problem with Firefox under linux. One way that I partially resolved it was with changing my gtk theme. Change to one that uses the generic gtk engine...not a fancy one like clearlooks or murrine or something. Some gtk engines are coded badly and lag a bit. Although it doesn't completely resolve the issue it speeds it up a bit...I have always wonder why firefox seems slower on linux than windows...I thought it was just me being crazy, but I guess not.