I've been using tabs since I first started using Firefox...back when Version was newly released. I've usually got 3 tabs open at the same time after opening my browser, one for Gmail, one for a few tech blogs I read and one for WC 
Occasionally it so happens that I want to read two BUMPED forum posts at the same time, so I middle-click on one of them to open it in a new tab while the other opens in the same tab....really, a lot like Zu's post.
If you do, is it because it's usefull or because it's just there?
It's useful because it saves taskbar space. I don't want my taskbar to have three FF windows open 
And not counting Google Chrome, using tabs can sometimes cause trouble. For example, if Firefox crashes with 3 tabs open, I lose all three tabs. But if it's three windows open, I lost the window that crashes. [Though with the session restore feature it doesn't matter anymore]
So my question is do you use the tab function?
Yes
I'm just starting to use the function in my browser
Really? What browser do you use?