Jared Wein, Senior Software Engineer at Mozilla, wrote on his blog praising the new Firefox tab closing feature:
"Why “close tabs to the right” and not “close tabs to the left”? When we open new tabs they appear on the end, and so naturally tabs that have a longer lifetime end up being promoted to the start-side of the bar. This leads us towards the situation where closing tabs “to the right” is a simple way of closing the ephemeral tabs."
Let me tell you the close tabs to the right feature already exists, it’s call Tab Mix Plus, don't use the inferior Firefox version. Amongst many other useful things via TMP you can close tabs to right OR the left. Perhaps the most useful TMP tool is being able to avoid clicking tabs to switch to the tab. Via mouse gestures you simply point to the tab, hover over it and it switches to the tab without any pesky clicking (I have the time to switch set to zero ms, instantaneous, but you can set a longer interval to hover-point switch if you want). Another good TMP thing is a list of recently closed tabs. The new FF option is a pale imitation of TMP.