I discovered a new shortcut in Firefox 1.5 today. In the previous version of Firefox, I used the SuperDragAndGo extension to open links in new tabs. It turns out that in Firefox 1.5, you can drag a link onto the tab toolbar and a new link will open automatically. I love it when Firefox integrates functionality from my favourite extensions (like miniT).
On a related note, dragging selected text into the search box in the Google toolbar will automatically bring up a Google search page for the selected text.