Superficially Firefox 3.5 might look a lot like its predecessors, but it boasts a number of major changes, ranging from under-the-hood improvements as well as some truly useful yet subtle feature tweaks.
Like IE8, Firefox 3.5 sports a private browsing mode. When using the private browsing doodad, Firefox gets Alzheimers, forgetting to record history, cookies, usernames, or passwords. What sets the Fox apart from the hounds is its ability to close all of the pages already open when private browsing is started, restoring them once Private Browsing is switched off. Nice.
Firefox 3.5 has also been taking geography lessons and can allow sites to determine your physical location using both your IP address and nearby Wi-Fi or mobile networks (depending on how you're hooked into the interweb).
Before you can utter "big brother", bare in mind that any site wanting your location must first get your permission. Given Forefox's huge developer community, expect this feature to be used in weird and wonderful ways going forward.
Mozilla has also copied Chrome by ripped into Firefox 3.5's tabs - literally. Tabs can now tear off to form new browser windows and be dragged and dropped to be rearranged. Both Chrome and Safari already support tearaway tabs as well as drag and drop rearranging in a smoother real-time fashion (Firefox 3.5 can only show you an outline of where dragged tabs will end up as you drop them). Best of all in the tab department has been the addition of a small + sign on the tab bar. It might not sound like much, but it makes creating new tabs far faster.
Firefox 3.5 also remembers what you've entered into web forms before you close the browser window, restoring information that you've entered should you re-visit the page.
Once again it's not a massive reality transforming feature but it does make a real difference. Like many, I've lost large and valuable chunks of my life re-entering data into web-forms after accidentally closing the page (or having my browser crash) because the browser couldn't remember it.
Combined with a bunch of genuinely useful features and thousands of themes, applications and add-ons, Firefox 3.5 will be pretty darned hard for other browsers to beat.