I am using Firefox 26.0 on most recent Ubuntu. Math formulas surrounded with the dollar sign are not being rendered as formulas, just as plain text. I can't pinpoint when it stopped working, because I am just an sporadic user of math enabled StackExchange sites, but some time ago they used to be displayed as nicely rendered LaTeX formulas.
Ok, I figured it out. HTTPS Everywhere extension was messing with my ability to have math rendered. Probably a bug on them...
When accessing a pages with MathJax via the HTTPS protocol, we now use the CDN specified by the MathJax FAQ:
How do I access the MathJax CDN using a https secure connection?
The MathJax CDN utilizes Rackspace’s CDN service. Unfortunately, this currently does not offer https support for the default address cdn.mathjax.org. However, as an alternative, you can use the following unmapped address:
While not as appealing as cdn.mathjax.org, this address is stable and safe to use. For more information, see our documentation.
So everyone should now be able to enjoy the beauty of quality mathematics notation whichever protocol they prefer.