While the awesome bar, download pause and resume, malware protection, the new themes, and serious performance improvements are perhaps the most representative features introduced with Firefox 3, here are some other useful ones you may not be aware:

Duplicate tabs. Press Ctrl while dragging a tab to create a duplicate of the dragged tab including its history. Note that this feature doesn’t work on Mac. Move tabs to another window. Just drag a tab and drop it on another window to move it. If the dragged tab is the last one in its window, it will be closed. Resize the search bar. When the search bar is placed next to the location bar, you can drag the handler between them (invisible on Windows and Linux) to resize it. Add keyword search. Select Manage Search Engines… in the search engines menu to open the Search Engines Manager, select a search engine, press Edit Keyword… and enter a few characters to define one. Now you can enter the keyword followed by your search terms in the location bar to search with that plugin. Discontinuous selections. Make a selection with the mouse as you usually do. Then press and hold the Ctrl (Cmd on Mac) key to make additional selections and create a larger discontinuous one so you can copy or print just what you need. Integrated add-ons. No need to visit Mozilla Add-ons because Firefox 3 brings it to you directly from the Add-ons Manager new Get Add-ons page: search, find and install with a single click. Disable plugins. Crashing? It may be a plugin. To be sure, disable the suspect through the new Plugins page in the Add-ons Manager. Check your crashes. Enter about:crashes in the location bar to get a list of crashes submitted by Breakpad (Mozilla’s crash reporter). Click on a crash report to get details provided by Socorro, Mozilla’s crash reports server.