Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

Please report back soon.

Try to reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl+0 (zero); Command+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

If you have made changes to Advanced font settings like increasing the minimum/default font size then try the default minimum setting "none" and the default font size 16 in case the current setting is causing problems.

• Firefox > Preferences > Content : Fonts & Colors > Advanced > Minimum Font Size (none)

Make sure that you allow pages to choose their own fonts.

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

I did everything suggested, all the way through the troubleshooting and extensions, etc. The problem still exists. Thank you for trying to help.

I did all the stuff suggested in the Troubleshooting section, after the easier fixes didn't work, and the problem still exists. Thank you for trying to help. I may become a Chrome person now.

This can happen if a new font tag gets opened that sets a relative font size like 1.2em or 120% and the previous font tag didn't get closed, so multiple font tags get nested. This used to be a bug in a previous Firefox version, but this should have been fixed some time ago if I remember correctly.

• http://www.google.com/search?q=site%3Asupport.mozilla.org+nested+font+tags

Are you using one of the large webmail sites (e.g., Yahoo, Gmail, Microsoft)? That might make it easier for a volunteer to test your exact conditions (e.g., same template, same settings, etc.).

Thank you everyone for trying to help. I've done everything suggested above, have been using yahoo mail, and nothing works. I uninstalled Firefox and reinstalled it with the same problem. I think I will have to use Chrome and Safari and get rid of Firefox altogether. Since it doesn't happen in other browsers, the problem must be with Firefox, and it rather chaps me that one is unable to contact anyone at Mozilla, by any method, to find out about or walk me through a possible fix. Same problem contacting anyone at yahoo. Grrr.

Well, you wouldn't believe it, but I decided to try everything one more time before uninstalling Firefox forever, and cor-el's "View zoom reset" worked! I don't know what happened between this time and last but it worked. Thank you!