There's an old thread about this somewhere, but it was basically thrown back and forth about the fact that while the logo shows "xF," the "appropriate" abbreviation is "XF."
If it's officially XenForo, then here's an idea-- in the logo try making the X a little bigger, but not as big as a full capital X. In height it would be halfway between the height of the lowercase letters and the capital F.
I think the confusion lies with many initial xenForo users coming from the vBulletin community which was very often shortformed as vB. With the official logo using a small letter for the first letter it seems like it was a deliberate attempt to solidify the short form as xF.