If you find yourself accidentally clicking the close button on firefox tabs quite frequently, you can remove that little “x” by following these instructions:
1. In the address bar, type about:config
2. Filter results by typing browser.tabs.closeButtons
3. You will see the current value set at 1.
4. Right click on 1 and choose Modify from the menu.
5. Selecting a value of 3, would remove the close button from individual tabs and put a close button on the right hand corner.
6. Selecting a value of 2 would remove the close button. You will still be able to close the tab by pressing Ctrl+W.