The <a href="http://www.gold-saucer.org/mathml/greasemonkey/display-latex.user.js">Greasemonkey MathML script</a> written by Steve Cheng and linked to in <a href="http://mathoverflow.net/questions/22141/how-do-i-see-latex-math-on-any-web-page/22248#22248">Scott Morrison's answer</a> worked only partially for me in Firefox on Windows 7: it did not display many \mathbb, \mathcal, and \mathfrak characters because they are missing in the fonts. Installing additional <a href="https://developer.mozilla.org/en/Mozilla_MathML_Project/Fonts">STIX and Asana Math fonts</a> did not help, in fact it made the display looking worse. 

So I rewrote the script (a long a tedious job finding the right Unicode codes and putting them in the right places). I also added arxiv.org and front.math.ucdavis.edu to the sites supported by default.

Here are the detailed instructions for the method that produces good results using Mozilla Firefox on Windows 7. I haven't tested on other systems, you are welcome to share your experiences in the comments.

 1. Click <a href="https://addons.mozilla.org/en-US/firefox/addon/748">here</a> to install the Greasemonkey Firefox extension.

 2. Download <a href="https://docs.google.com/leaf?id=0B_3iWUtnyGylY2I5MmFjOTItZWQyOC00YjU0LThkOGYtYzFjZDQ4NGZhYzdi&hl=en">this modified Greasemonkey script</a> and save it on your Desktop.

 3. In Firefox menu, File > Open File, navigate to the downloaded script and open it. 
Greasemonkey will offer to install it. Do that.

That should be it. Check how it works by looking at some arXiv abstracts such as 
<a href="http://arxiv.org/abs/1004.2614">this</a>, 
or <a href="http://arxiv.org/abs/1004.4188">this</a>.

Even when the authors use custom notations, such as \red or \cA, removing the dollar signs, putting math in a different font, and using sub- and superscripts dramatically increases the readability in my experience. Enjoy!