Firefox 11 doesn't show any text on some sites as Google and Youtube. Also Lotus Symphony doesn't show many UI Labels. This problem occurred suddenly.
Firefox display error:
Symphony display error:
It seems that you have a problem with your system's fonts. Try reinstalling the font. You can see a guide to this here.
You can also try this:
This post seems very similar to yours (I would say is duplicated).
This is a problem with your fonts. I had this problem on different machines.
To solve it I had to set the appropriate rights of the fonts and regenerate the font cache. You can use the following script to fix the display of fonts in Firefox.
Take a look under the Firefox Preferences -> Content and see what Default Font is set there, like in the screenshot.
Now see if you have that font installed in Ubuntu System. If not (which i think is not the case, since common applications in general use common fonts, that come with the installation) either install it afterwards, or use another font as default in Firefox.
Also you might want to take a look under the Advanced-Button -> Default Character Encoding and make adjustments there! To me it seems, that your Encoding is wrong which is the problem if not mistaken!
If that still does not resolve your problem, please edit your post and add a screenshot and/or more information about your settings that you use in FF.