I haven't noticed it, so I took some screenshots. Here's how Glyphicons [1] render for me (FF 27.0.1, Mac):

http://snappy-app.com/s/show.php?pass=2423776c8599b57c05dc4e...

Enlarged:

http://snappy-app.com/s/show.php?pass=a1808d59f6efff53d1c847...

Am I missing something?

[1] http://getbootstrap.com/components/