Although I use Firefox for quite a while now, now and then you still learn useful things. For example, the usage of the middle-mouse-button (or more commonly: clicking the scroll wheel). Not long after I started using Firefox 1.5 and I couldn’t use the Tabbrowser Preferences extension anymore I learned that clicking on a tab with your scroll wheel closes it. A very nice feature indeed (before that, I right clicked on the tab to close it).
It was only last week when I discovered another nice feature: when you click with your scroll wheen on a link (either on a page, or on a bookmark), the page opens in a new tab. Another time saver! (before that, I clicked right to open a link in a new tab, or – more recently – Ctrl-clicked it).