1. Goto tools -> options -> tabs.
2. Make sure "always show the tab bar" is enabled.
3. Now, make sure you have only one tab open and make sure it's blank.
4. Type something in the address field (but don't press enter).
5. Click the X on the tab, or right-click on the tab and click "close tab", or middle-click on the tab etc.
Firefox will basically close the tab and open a new one, which will give you a nice, blank address field. (It may just clear the address field in this case for efficiency, but the technicality is not important.)
Now, try the same thing in IE7 and Safari …
Oops, there's no close option for the tab, even if you have a page loaded in it.
Now, in Opera:
1. Goto tools -> preferences -> advanced -> tabs.
2. Make sure "allow window with no tabs" is unchecked.
3. Make sure "Click on tab to minimize" is checked.
4. Make sure "Show close button on each tab" is checked.
and try the same …
Oops, the same thing as IE and Safari. The close functions are disabled for the tab (unless you loaded a page in it, then it works like Firefox).
Now, I can guess why IE, Safari and Opera do what they do. Technically, if you're not allowed to have a window with no tabs, it doesn't make sense to have a close function when you only have one tab. Also, technically, if you want to go to a site in the current tab, you'd clear the address field, type in the URI and press enter or click GO etc. (or click in the address field, which selects all the text and start typing). In short, Firefox's way doesn't technically make sense.
However, the reality is, Firefox's way of providing a close function and having it clear the tab is totally awesome. It may not make sense, but it's better than what the others do. It's much more pleasant to start typing in an empty tab and using the close function to clear the tab this way is so convenient, cool, exciting and fun.
I think this is a case of IE, Safari and Opera trying to make too much sense.
Luckily, there's hope for Opera. As mentioned above, if the tab isn't blank, then you can indeed use a close function to clear the tab like Firefox. Even better, turn off "show close button on each tab". That will give you an MDI X button near the top right of the window. This close button will clear the tab just like Firefox (even if the tab is totally blank).
On a side note, the MDI X button has other special powers. If the regular close functions fail to close a page (because the page is freezing up and messing with Opera's UI), click the MDI X button. It'll zap the tab for you and save you from having to kill Opera.