One of the most notable features added is the removal of the display of a site's favicon in the area next to the URL. It is now replaced with a globe for normal HTTP sites, a gray padlock for HTTPS sites, and a green padlock for HTTPS sites that has fully identified certificates. You can still view a sites favicon in the tab bar, though.
This change was already coming for those that were wondering, it is to stop trickery to normal users. Sites can make their favicon a padlock, making a person feel that the page is secure. Most users however do not pay attention to the URL bar as well, and notice that the site is just standard HTTP. This change is now here to prevent social engineering attacks, and other stuff like that.
Another interesting but needed feature is HTTPS (secured) Google Searches. You can now send search queries to Google using HTTPS encryption, even if you are not logged in. Encrypting searches allows better control of their data. To see what I mean, go here and refer to the 2nd and 3rd paragraphs.
Update
To update Firefox, you can go to the About Firefox menu and click check for updates. If you are using a Linux distro such as Ubuntu or Linux Mint, the update should be arriving in your update manager shortly, along with updated Firefox add ins such as Global Menu bar integration (for Ubuntu and its Unity interface).
Let me know of your thoughts below.