Hi

This article should help you to close tabs in Firefox for Android.

I hope that this helps, but if not, please come back here and we will see what we can do to help.

If you are viewing a tab that you just opened (i.e., long press a link then Open the link in a new tab), pressing Back will close that new tab.

Otherwise, if the tab has a lot of history, it would be very annoying to go back, back, back..