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.

Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1

Its already turned on

It is possible that the page use a font that is downloaded from the server and that it takes some time to arrive.

What happens if you temporarily disable downloadable font as a test by setting gfx.downloadable_fonts.enabled pref to false on the about:config page?

The above error happens more often when I set it to False

Yes, that is what I would expect. Firefox seems to have a problem with the (default) font that is initially used when the page is loading.

You can right-click and select "Inspect Element" to open the Inspector ("3-bar" menu button or Tools > Web Developer) with this element selected. You can check the font used for selected text in the Font tab in the right pane of the Inspector.

• https://developer.mozilla.org/Tools/Page_Inspector
• https://developer.mozilla.org/Tools/Page_Inspector/How_to/View_fonts

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.

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

I fixed it by installing fonts-lyx package on ubuntu . Actually i uninstalled firefox and tried to re-install it . Then the above package came up in suggested packages. So I installed it manually. From then the firefox did not show any above errors.