Bre901, you are very correct, indeed it is an extension!
Not being familiar with Opera, I was quite happy with FF's tab options when I first used it (actually I used the Mozilla browser before FF 1.0 was released) and I still am, for me the Tabbrowser preferences extension does the trick. I can see that the Tab mix extension offers some more options for those who want to customize their browser a bit more.
My guess is that FF is aimed predominantly at those computer users wishing to move away from IE which is why in its 'basic' mode it mimics a lot of things from IE (a lot of people are confused when confronted with Tabs for the first time). The added value in using it is that it is easily customized to suit people's needs. Therefore I can run it with the settings I like, but someone else can use it as an IE clone.
My main reasons for switching to FF are still: less buggy than IE, and less susceptible to spyware. I would have been happy with just that, so all the extra's are a bonus!