I sometimes find that if I've been browsing something big (maybe a page with lots of big images, embedded video etc) and then close down (because browser is being too slow) I find I have a browserd still running which is very large, taking up memory (at least as reported by listing processes.) sometimes just killing it and letting it respawn when I launch a new browser is good enough. Sometimes I find a healthy reboot to ensure things are cleared up is better.