Thanks to a teenage Firefox aficionado from Tasmania, Australia, you can make your Firefox 3.5 or 3.6 look like mock-ups we've seen for Firefox 3.7 and Firefox 4.0.
To get a peek at one potential future for Firefox, install the Strata40 theme and the experimental StrataBuddy add-on. Like one of those classic Superman comics that had Supermen from across time teaming up to defeat a dastardly villain, the add-on provides the interface tweaks while the theme gives Firefox the look you're looking for. Several other add-ons are also recommended to complete the experience: AppTabs, which is similar to FaviconizeTab and lets you shrink a tab down to its favicon; Fission, which merges the progress meter into the address bar; and Tab Progress Bar, which sticks a progress meter in each tab. Users should note that the key feature in Tab Progress Bar is also available as one of many in Tab Mix Plus, and that the two add-ons are incompatible with each other.
When you install Strata40 and StrataBuddy, you'll need to go into the StrataBuddy options to activate key features. These include the Style and Background Style choices under the Strata tab, the tab style under Tabs, and the font and progress meter changes under Miscellaneous.
While I've been using Download Statusbar for several years, I prefer to minimize the number of add-ons I'm running. The Omnibar and Hide Menubar features can be easily enacted by removing your search box and hiding the Menubar. I also hide the Statusbar, and use URL Tooltip to reveal links when I mouse over them. I prefer to use Personal Menu, as well, for providing an extra level of customization to what used to be the Menubar.
Personal preferences aside, this is an excellent visual tweak to make Firefox 3.5 and the impending Firefox 3.6 free up screen real estate and feel a bit more futuristic. On a related note, Firefox evangelist Mike Beltzner said in an interview today that he expects Firefox 3.6, currently available as a release candidate, to be finished by the end of January.