Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

You can check for corrupted and duplicate fonts and other font issues:

• http://www.thexlab.com/faqs/multipleappsquit.html - Font Book 2.0 Help: Checking for damaged fonts
• http://www.creativetechs.com/iq/garbled_fonts_troubleshooting_guide.html

You can do a font test to see if you can identify corrupted font(s).

• http://browserspy.dk/fonts-flash.php?detail=1

You can try different default fonts and temporarily disable website fonts to test the selected default font.

• Firefox > Preferences > Content : Fonts & Colors > Advanced
  [ ] "Allow pages to choose their own fonts, instead of my selections above"

sajm004 said

I now find that when I do a Google search in firefox the search result text is bold. This has only just started to happen. To try to solve the issue I have updated firefox, uninstalled and new fonts that I have installed, cleared history and cookies and reset my zoom. Would love to hear any suggestions as it is really frustrating it only seems to happen with google not on any websites. Also I'm on a Mac and I have upgraded to OS X Yosemite.

thanks for the suggestion I have just checked and "Allow Web Sites To Choose Their Own" is already turn on