It is ok. It is not a bug. Middle click of the mouse is intended to close tabs.

• See Mouse Shortcuts
• you may also be interested in keyboard shortcuts and accessibility

But the tab App unusual tab, it should not be closed by clicking the middle mouse button or I not right?

You have discovered a common feature of windows apps.

In many programs, middle-clicking on something (such as a link in Firefox, or on the icon in the taskbar in Windows 7) will open it in a new tab.

Middle-clicking on a tab-like object will generally close it.

Unfortunately, I see no option in about:config that will let you change this behavior. Either my search-fu is weak, or this might be a good feature request. ;)

>You have discovered a common feature of windows apps.

I use middle mouse button for close tabs a long time, but Opera browser not close fixed tabs by this way (now specifically downloaded, install, and tried).

Think this might be a good feature request or an a bug ?

You can use a tab related extension to lock a tab and prevent an accidental closure.

• Tab Utilities: https://addons.mozilla.org/firefox/addon/tab-utilities/
• Tab Mix Plus: https://addons.mozilla.org/firefox/addon/tab-mix-plus/

why I need extension? Firefox 4 already have this feature! but not prevent an accidental closure. =(

While tab protection isn't a standard feature of Firefox (you would need a plugin like those listed above), you can recover closed tabs.

Hitting Ctrl+Shift+T will recover the last closed tab (you can keep pressing it to bring back older tabs). You can also check History>Recently Closed Tabs for a list of tabs you closed.

It doesn't recover the exact state of the tab (it will reload the page as if you hit Refresh, which might clear forms or require information to be resent), but it does recover the tab's history.

Thank you. I already requested this feature for next release.

Thanks all!

+1 I think the best action when you click middle mouse on app tab is to duplicate it. While click on regular tab will still close it.