The newest version of the Firefox web browser was released on March 22nd, and since by default Firefox is configured to automatically check for updates, you may already be using it. If not, you can upgrade to Firefox 4 by clicking this link; when the install process is complete you'll just need to restart Firefox.
I usually download a new version of Firefox as soon as its available, but have gotten "burned" because many extensions & add-ons I use aren't immediately compatible -- developers usually need a couple of days to update an add-on to work with the latest version of Firefox.
If you use the Firefox web browser, you can enhance it by installing the free Adblock Plus add-on. After you install this add-on, you can have it block all those annoying advertising images and banner ads you see on web sites.