Make sure that you do not drag a tab in the browser window.
Firefox 3.5 versions and later have a feature called tear-off tabs.
You can detach a tab from the current window and open it in a new window by dragging a tab in the browser window.
You can drag that tab back to the tab bar in the original window to undo that detaching.

No , I am not dragging the tab .. it happens when i scroll the mouse .. irrespective of the position of the cursor .. It happens when the browser seems to be over loaded , i mean say a page with huge number of images are loading in one tab ...and when i switch to another tab and try scrolling down the page , the tab dettach or say moves as separate window..

I'm having this same issue, though it happens a bit differently from the way it happens to naveen_reloaded. I can never seem to repeat it the same way every time, but what seems to happen is when I have a few tabs open, and middle-click to open another tab, this happens. But what's strange is that it will not always open into a new window until I click on the new tab. I know about dragging the tab down and it opening into a new window, and I am making sure not to do that. However, this is still happening.

No such problem here (ever, since Mozilla 1.x)