firefox firefox. its been rated as one of the most secure browser.

HOWEVER, in terms of memmory usage, Opera does better.
Dont believe me?

open 20 tabs with yahoo.com in firefox and opera
ctrl alt del - to get the task manager

Look at your process - look for opera and firefox
you'll be amazed at how much memmory opera saves you

besides, you dont have to re-open sites on opera. theres a "use last session" option where it opens back all the sites you last visited...

so yeah... both these browsers are just great