It is possible that you move the mouse a little bit downwards while clicking the tab and that can cause the tab to get detached and opened in a new 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.

I don't believe I moved the mouse downward. Does not happen all the time. Will see if I am at fault next time it happens and will report back.

... i have the same problem... and until now, i can't reconstruct how it happens.... the phenomen is just quite arbitrary...

Sorry but it is constant, I just click once, do not shift the mouse and it just opens the tab in a new window, I am sooooo careful now clicking tabs. And I see I am not alone.

I had this happen as well, but it was in Firefox 3.6. ^^

And I believe it is a slight finger movement that's difficult if not almost impossible to detect. That's what it seems to be in my case, anyway. :S

Gosh, then the finger movement must be almost not non-existant as you just click it and even when I am very careful it still opens a new window.

Strange, never got this with the other versions but getting with this, it is very annoying, IE9 does not do this nor Chrome, just strange.

i'm only getting it with firefox 4, too... and never got it in other versions... but it's nice to see that i'm not alone with this problem... i just began to think that my tab-clicking is something like a nervous twitch and i made a double click instead of a single... O.o

No not a twitch, it just goes off on you. Hope they get this corrected. Till then back to IE and Chrome.

Guess I'm getting the same thing. When I click on a site, the window jumps and noticed that it opens in a new window underneath the window I'm clicking on.

At first I did not know it opened in a new window underneath the main window then would lick on the link again then when getting off that link I would return to the main window then when closing the main window I noticed that the site I first clicked on had opened underneath this main window.

Yes, I know there is nothing perfect, but IT IS kind of annoying. This only started happening with the new version of FF.

@mlsto, I don't doubt that it is caused by a bug in cases when you're being very careful, I'm just saying it can also be caused by a small finger movement that you're not conscious of.

I uninstalled FF and deleted all preferences and reinstalled and now everything seems to be working. I believe it had something to do with upgrading, it seems better to do a clean install.

Hope this helps.

Firefox 4 crashes often and is very slow. Very slow. I have the Ladybug add-on. From the beginning Foxfire4 has been a problem. Should I delete and reload. I really do not want lose my Bookmarks. REHKSH

Why not save your bookmarks - FireFox tab at top > Bookmarks > Import and Backup