On tabbed browsing, Firefox will not only warn you when closing several tabs and windows at once but will prompt if you want to save the currently open tab set: a good catch and a good way to introduce this helpful feature to new users.
Tabs now scroll with a smoothly. You can also duplicate tabs when you press Ctrl while dragging a tab and move them across windows by dragging them. Unlike previous betas, you get actual duplicates including the complete tab’s history. A very helpful addition!
Firefox can save site specific preferences. For example you could allow just some pages to use AJAX, set a specific spell checker language, images, etc. Firefox 3 will remember the zoom level you have applied for a certain site and will zoom automatically the next time you visit it.
Net protocols (like irc, news, webcal and mailto) and certain documents handling can be passed to web services. For example, Yahoo Mail or Gmail may become your default application for mailto: links. For this, the previous web feeds page has been extended to the Options window for all content and protocols. Firefox 3 recognizes podcasts and video podcast feeds from regular ones so you can specify a different application for each.
Media feeds are displayed with a link to the media file in feed preview mode.
The search engine manager lets you set and change keywords for search engines. With keywords, if you associate the w keyword with Wikipedia (now a default search engine), you could enter w hawaii in the locationbar and search Wikipedia for Hawaii.