The built-in OLPC browser is basic but functional. The Browse tab has forward and backward buttons, reload and a box to type in URLs.
While the XO's browser may use the same Gecko engine as Firefox, this is no Firefox. It doesn't support keyboard shortcuts for backwards and forwards. It doesn't support tabs, or the ability to open a link in a separate window. This makes browsing feel constrained, as one can't open a link in the background and come back to read it later. You either click it now and remember to come back to the original page, or read the original page and try to remember to follow the link later. This alone makes the browser frustrating to use.
It also lacks a familiar bookmark system. To bookmark a page, select the Browse tab and click the star in the upper right corner, and it will add a thumbnail view to the Tray at the bottom of the page. To hide the thumbnails, press Alt and Space. Next click the Activity tab and name the bookmark collection. Note the Share With option can allow others to watch a browsing session. Finally, click the Keep button to save the bookmark collection to the Journal.
After using the browser for a while the weird bookmark system and lack of tabs gets increasingly annoying.
Fortunately, Opera has a build for the XO, which supports traditional bookmarks and tabbed browsing. Opera makes full use the XO's high resolution display and packs a lot of information on screen in crisp but tiny fonts. It's a full port, so it supports the +/- keys to change the font size. The system fonts for dialogs are microscopically small.
Firefox and other Linux apps can be installed, but non-native XO apps aren't integrated with the Sugar UI and must be launched from a terminal window. Hopefully, a native Firefox port will be available soon.