button not working

[agdesign]

I have four social media buttons on my webpage that aren't being activated properly.

http://www. andreagriecodesign .com/test/about.html

When viewing the page on Firefox, they only become activated when the cursor is dead center of the button or below (to the bottom edge of the button).

Any suggestions why and how to fix? I don't have this problem with any other images on any other pages.

Thanks