I installed the latest Firefox 4 beta 9 browser on Ubuntu and it shows promise. While it appears faster than v3.613, Chromium is still faster. Perhaps Mozilla will continue to tweak and refine its performance in the coming months. But it does offer a lot of customization options. In addition, the tab groups function is a nice feature.
Below are my screenshots: Below are tabs on top with Bookmarks bar, and without the Menu bar.
Below is shown tabs on bottom with the Bookmarks bar, without the Menu bar. This is similar to the current Firefox layout.
Below is shown Tabs on top, with the Menu bar, and the Bookmarks bar.
Below is shown a clean look with Tabs on top, without the Menu bar or Bookmarks bar.
You can get the latest Firefox 4 beta here: http://www.mozilla.com/en-US/firefox/beta/