There has been so many discussions about where to place the close-buttons for tabs that Mozilla has created a config key for this specific option.
To access the key, open a new tab, enter
about:config then press enter. In the filter search for
browser.tabs.closeButtons. Change this value according to your choice and the following table:
|0||Close-button only on the active tab|
|1||Close-button on all tabs|
|3||A closebutton at the end of the toolbar, not on the tabs|