It might be that you have adblock + or some other blocking software affecting the X. I do have adblock + and can simulate the problem by right clicking on the x and adding it to the filter. If I do, I lose the option to close the box. BTW, FF is at 23.x and not 24 unless you're using a beta version.
How do I report a problem with CNET and Firefox 24 not rendering web pages properly?
How do I fix this problem?
Example - When viewing CNET on IE there is a rectangle on the bottom center of the page that says CNET with F like - 1.2m - Follow CNET and g+ in this box.
There is an "x" to close it.
But when viewing this using Firefox you only see the g+ and you can not close this popup because the "x" is not usable.

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