This is happening quite often in Firefox at least in Ubuntu but I think also in PCLinuxOS. I open a few sites and all of a sudden for no apparent reason I cannot close the tab either by middle clicking on it or pressing the X or right-clicking and choosing Close Tab. The only way I can close it (or them) is to close FF and upon reopening it using one of the 3 options mentionned. As it happened to any of you and did you find a way to correct the problem.