Why does firefox show a shadow beneath my CSS shape?

Why does firefox show a shadow beneath my CSS shape?

References

Reference NO 1

http://stackoverflow.com/questions/13582026/why-does-firefox-show-a-shadow-beneath-my-css-shape

Reference NO 2

http://stackoverflow.com/questions/9450733/css-triangle-custom-border-color

Reference NO 3

http://css-tricks.com/triangle-with-shadow/

Reference NO 4

http://css-tricks.com/better-tabs-with-round-out-borders/

Reference NO 5

http://webdesign.about.com/od/csstutorials/a/aa102306.htm

Reference NO 6

http://designshack.net/articles/css/the-lowdown-on-before-and-after-in-css/

Reference NO 7

http://www.the-art-of-web.com/css/border-radius/

Reference NO 8

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