Which button are you clicking? Avoid the "red X" at the upper right. There should be one on the individual tab, but maybe if you have a ton of tabs open they are getting dropped?

Here are some workarounds:

(1) right-click tab and choose Close tab to close that tab

(2) Press Ctrl+w to close the currently displayed tab

Are you clicking on the X on the Title bar? as this closes the window and not the tab that was open in window.

With a mouse that has a scroll wheel you can use the scroll wheel (middle-click) on the tab to close it. There is a X on right side of the tabs depending on how many are open that you can use to close it also.

https://support.mozilla.org/en-US/kb/tabs-organize-websites-single-window

You can set the browser.tabs.closeWindowWithLastTab pref to false on the about:config page to prevent closing the last tab from closing that window and make a close button appear if only one tab is open.

• http://kb.mozillazine.org/about:config