I had problems with www-client/mozilla-firefox-3.0-r1 rendering vertical lines in MathML content as clubs suit symbols, e.g. example 24 from http://www.mozilla.org/projects/mathml/demo/texvsmml.xhtml. I also created my own little problem description page at http://martin.von-gagern.net/howtos/20080713-ff3-mathml which contains a screenshot of how such a broken formula looks like on my system.
The cause of the problem seemed to be my customization of the font.mathfont-family setting in firefox, which I had performed in response to the pkg_postinst message in the media-fonts/mathematica-fonts (e.g. version 6.0-r1) ebuild. I suggest you either drop this suggestion, or even tell users that it might in some cases be a good idea to remove that setting again.
Thank you for report. But I failed to reproduce your problem. I've set everything as it was suggested in elog messages but page renders correctly. What other fonts you have installed on your system?
(In reply to comment #1)
> Thank you for report. But I failed to reproduce your problem. I've set
> everything as it was suggested in elog messages but page renders correctly.
If someone would point out a location in the mozilla sources where the actual font selection for these lines happens, I might try to debug my firefox to see what font it actually uses. So far I haven't found the right location myself.
> What other fonts you have installed on your system?
$ ls -1 /var/db/pkg/media-fonts/
Martin, it is media-fonts/texcm-ttf fonts which caused you troubles, and since upstream  tells us not to use CMSY10, CMEX10 fonts new browsers, I've modified our instructions...
Thank you for great report which is now FIXED.