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 the corresponding Unicode characters were 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 and tedious job finding the correct 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_3iWUtnyGylYTc3YjMxNmMtNTgyYS00ZjdkLThmY2EtOTRhYjU2MWZiYTYz&hl=en">this modified Greasemonkey script</a> and save it to your Desktop.

 3. From the Firefox menu bar, 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 \cE, removing the dollar signs, putting math in a different font, and using sub- and superscripts dramatically increases the readability in my experience. 

**Edit:** I also fixed the displayed formulas with double dollars, which the original script did not handle correctly. So now you can also view <a href="http://arxiv.org/abs/0903.1879">this</a> and <a href="http://arxiv.org/abs/1004.4623">this</a>.

So in the end this was more of a community service than a question. Enjoy the results! Unless you are one of those who voted to close this question or its parent. Then don't use it -- if you had your way, you would not see this.