the common Windows Ctrl-F4 way of closing documents is dangerous. It’s easy to confuse Ctrl-F4 with Alt-F4, which closes the application. I never use Ctrl-F4 because of this danger.
Ctrl-W is the same as Ctrl-F4 in most Windows applications, including Firefox.
You can also close tab with mouse middle click (wheel click).
Close button on tabs is unnecessary and is a waste of space.