The character in question is ಠ (U+0CA0; ಠ). Here are three screenshots:
Chrome 17 for Mac

Firefox 7 for Mac

Firefox > 4 for Windows

All browsers I tried had UTF-8 as encoding. Here it is copy-pasted : ಠ_ಠ, but I have no idea how you are seeing it.
This is probably due to the different platforms and browsers having different default fonts and font implementations.
The
font-familyon SO is:So different fonts will apply on different platforms.
In your Mac examples, the different browsers display a “missing” glyph differently – Chrome with a simple square, FF with the hex Unicode of the missing glyph rendered within the box (in this case
0CA0).