Which graphics display card do you have?

If you have an ATI card then try to disable "Morphological Filtering" in the Catalyst display driver:

• Catalyst Control Center > 3D Settings > Disable Morphological Filtering

Does it help if you disable the website fonts and try a different font?

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

Maybe you can identify the font that is causing the problems.

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

You will likely wind up with freezing issues too... go back to 3.6.2 - I just did, and I am now happy with FF again.

I have the same problem it started just recently, don't really know what triggered the font bold problem.

@cor-el: I have NVIDIA GeForce G105M... When I check the about:support in FF, it says "ClearType-parameters - ClearType-parameters niet gevonden" (ClearType-parameters not found). Can this be causing the problem?

I can change the font, but I don't want that. I want to be able to see the site's original style. I am a webprogrammer, so changing the font in a browser is no option for me.

I tried that URL u gave me, but I don't find any font that looks weird... Also the fonts declared in Facebook's stylesheet (font-family: 'lucida grande', tahoma, verdana, arial, sans-serif;) are displayed normally.

@MitchBentley: That was my decision as well when I first upgraded to FF4.0. But I'd like to make use of the new features and technology as I make websites myself...

I think I've found what the problem is... When I check about:support, I get some information, including:

 Toepassingsbasics

       Naam
       Firefox

       Versie
       6.0

       Useragent
       Mozilla/5.0 (Windows NT 6.1; WOW64; rv:6.0) Gecko/20100101 Firefox/6.0

       Profielmap

         Bijbehorende map openen

       Ingeschakelde plug-ins

         about:plugins

       Buildconfiguratie

         about:buildconfig

 Grafisch

       Adapterbeschrijving
         NVIDIA GeForce G105M
       Vendor-ID
         10de
       Device-ID
         0a68
       Adapter-RAM
         512
       Adapterstuurprogramma’s
         nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um
       Stuurprogrammaversie
         8.16.11.8817
       Datum stuurprogramma
         11-28-2009
       Direct2D ingeschakeld
         Geblokkeerd voor uw grafische stuurprogramma. Probeer uw grafische stuurprogramma bij te werken naar versie 257.21 of nieuwer.
       DirectWrite ingeschakeld
         false (6.1.7601.17563)
       ClearType-parameters
         ClearType-parameters niet gevonden
       WebGL-renderer
         Geblokkeerd voor uw grafische stuurprogramma. Probeer uw grafische stuurprogramma bij te werken naar versie 257.21 of nieuwer.
       GPU-versnelde vensters
         0/1. Geblokkeerd voor uw grafische stuurprogramma. Probeer uw grafische stuurprogramma bij te werken naar versie 257.21 of nieuwer.

A lot of my Graphics settings are disabled ("geblokkeerd") for Firefox... Why is this? How can I solve this?

UPDATE: I am now trying to download a driver update for my Graphics...

NVIDIA Driver Downloads

Product: NVIDIA GeForce G105M
Current Installed Driver: 188.17
Latest Driver Update:

 Verde 280.26 Driver
 Version: 280.26  WHQL
 Release Date: August 9, 2011

Edit: The update didn't help.. :/

After searching for a while on my own website (which has the same problem), I found out the font Lucida Grande is the one showing that bold and unreadable. If I change it to any other font, i shows up normal.

What can cause this only to happen with the Lucida Grande font?

font-family: 'Lucida Grande';

Triple post, I'm sorry.. BUT: I have found the solution!

As soon as I knew what font was causing the problem, I searched for more information about the font.

Lucida Grande seemed to be an older font, not used by Windows.. But it gets installed with Safari etc.

The font files of Lucida Grande I had, probably were very old. I deleted them and downloaded other ones. I installed those and my problems got fixed.

The font files I installed: http://font.pendownload.com/2009/10/lucida-grande-facebook-font/