One feature that affects fonts is hardware graphics acceleration. Could you try disabling it?

You usually need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

orange Firefox button or classic Tools menu > Options > Advanced

On the "General" mini-tab, un-check the box for "Use hardware acceleration when available"

If you restart Firefox, is the issue resolved or improved?

Which graphics display card do you have and which driver version are you using?

This can also be a problem with a missing or corrupted font.

You can use this extension to see which fonts are used for selected text.

• fontinfo: https://addons.mozilla.org/firefox/addon/fontinfo/

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.

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

Indeed, When I disabled the box : "Use hardware acceleration when available". The font render was OK : No more "bold" effect.

But I don't understand that, because before 2 days, the render was OK too.

Here my Firefox information, with a screenshot :

 Paramètres de base de l'application

       Nom
       Firefox

       Version
       15.0

       Agent utilisateur
       Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20100101 Firefox/15.0

       Dossier de profil

         Ouvrir le dossier correspondant

       Plugins activés

         about:plugins

       Configuration de compilation

         about:buildconfig

       Rapports de plantage

         about:crashes

       Utilisation mémoire

         about:memory

 Extensions

       Nom

       Version

       Activée

       ID

       Adobe Acrobat - Create PDF
       1.2
       false
       web2pdfextension@web2pdf.adobedotcom

       Adobe Contribute Toolbar
       6.1
       false
       {01A8CA0A-4C96-465b-A49B-65C46FAD54F9}

       Skype Click to Call
       6.0.0.10297
       false
       {82AF8DCA-6DE9-405D-BD5E-43525BDAD38A}

 Préférences modifiées importantes

     Nom

     Valeur

       browser.cache.disk.capacity
       1048576

       browser.cache.disk.smart_size.first_run
       false

       browser.cache.disk.smart_size_cached_value
       593920

       browser.places.smartBookmarksVersion
       4

       browser.startup.homepage_override.buildID
       20120824154833

       browser.startup.homepage_override.mstone
       15.0

       extensions.lastAppVersion
       15.0

       gfx.direct3d.prefer_10_1
       true

       network.cookie.prefsMigrated
       true

       places.history.expiration.transient_current_max_pages
       104858

       privacy.sanitize.migrateFx3Prefs
       true

 Accélération graphique

       Description de la carte
       NVIDIA GeForce GT 335M

       ID du vendeur
       0x10de

       ID du périphérique
       0x0caf

       RAM de la carte
       1024

       Pilotes de la carte
       nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um

       Version du pilote
       8.17.12.8562

       Date du pilote
       10-15-2011

       Direct2D activé
       true

       DirectWrite activé
       true (6.1.7601.17789)

       Paramètres ClearType
       Gamma: 2200 Pixel Structure: RGB ClearType Level: 100 Enhanced Contrast: 50

       Rendu WebGL
       Google Inc. -- ANGLE (NVIDIA GeForce GT 335M   ) -- OpenGL ES 2.0 (ANGLE 1.0.0.1041)

       Fenêtres avec accélération graphique
       1/1 Direct3D 10

       AzureBackend
       direct2d

 JavaScript

       Ramasse-miettes incrémental
       0

 Versions des bibliothèques

       Version minimale attendue

       Version utilisée

       NSPR
       4.9.1
       4.9.1

       NSS
       3.13.6.0 Basic ECC
       3.13.6.0 Basic ECC

       NSS Util
       3.13.6.0
       3.13.6.0

       NSS SSL
       3.13.6.0 Basic ECC
       3.13.6.0 Basic ECC

       NSS S/MIME
       3.13.6.0 Basic ECC
       3.13.6.0 Basic ECC

Here are few tests :

1. With hardware graphics acceleration ON, fontinfo give : Arial Bold on wikipedia. Look the screenshot.

2. If change the sans-serif font in Tools > Options > Content : Fonts & Colors > Advanced. The result depend on the font. Sometimes, fontinfo give font_name Bold or just font_name.

3. So I looked with http://browserspy.dk/fonts-flash.php?detail=1. This website points out that some font doesn't work properly in Mozilla Firefox. Look the second screenshot with my entire font list at http://hpics.li/baf4e96 fontinfo show that some font are render with Bold (I didn't test all the font !) :

• Aharoni
• Arial
• Charlemagne Std
• Familiar Pro
• Magneto

4. I disabled "Allow pages to choose their own fonts, instead of my selections above" and the result (conditioned by the default font) is the same as the test n°3.

So maybe, I have problems with font files but I can't check that. I have Windows 7, and when I went in Windir/Fonts, the explorer doesn't return me the list as file. Instead of, it return me the font manager.

Thanks for your responses.