Firefox is pretty sluggish under ubuntu for some reason.
It's really slow with lots of windows open too. I suspect this might be related to the poor performance of flash, but it seems to poll each window at a set interval, lagging the entire system.
Opera is way faster under Ubuntu, except for flash and javascript though. Those have pretty abysmal performance, but it works better on non-animated web pages and having lots of windows open. I suspect firefox's issues could be due to the older version included with ubuntu. It might be worthwhile to uninstall it and manually install a .deb of the latest version. The firefox auto-updater will take over, rather than using synaptic.
Also, I think Ubuntu uses the open source version of Java by default. It's likely slower than the commercial version. (note, javascript has no relation to java...though newer versions of firefox bring vastly improved javascript performance, thus the recommendation of the .deb)