Odd bug with "Close Tab" x on Firefox when linked from LoL

Not sure how to describe it really, but to lay it out: I use Firefox, and when clicking a link from the game (like the one for Kled's introduction) it opens in Firefox (pretty much what I expect), in a new tab. If I don't have a Firefox window already open then the "X" in the corner of the tab (not the corner of the window) changes to a smaller thing with red around it (maybe it's the base skin, I do have a theme on my browser).

So, report with Firefox right? Thought about that, but checked and it's only happening when it's a link from the League Client, and I don't remember it happening before 6.15. There's problems with it crashing to... sounds like someone made some major changes when they shouldn't have.

Edit: Here's an image with an idea. Just not sure what type of thing could cause this. Not a big issue, but maybe it's related to something else. http://imgur.com/a/24TEv