Reset the page zoom on pages that cause problems: View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

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

Try to toggle some Boolean gfx.font_rendering prefs on the about:config page to disable some features and maybe set gfx.direct2d.disabled -> true to disable Direct2D.
Filter: gfx

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.

Still nothing :/ Ctr+0 it was first thing I've done.

Setting gfx.direct2d.disabled -> true makes no difference. The only setting that affect the way pages are displayed is gfx.font_rendering.directwrite.enabled. When this setting is toggled it is a bit better. Fonts on pages are more smooth but unfortunately it is far from perfection.

You can try this extension:

• Anti-Aliasing Tuner: https://addons.mozilla.org/firefox/addon/anti-aliasing-tuner/

Thanks for your help but i think the root cause of this problem is somewhere else. I'm trying to tune firefox by adjusting different settings but the effect is always poor.

Few days ago something happened and my firefox started to render fonts incorrectly on some sites. It was beta version. I though "It's beta, maybe they screwed up something..." so I removed FF from my computer (including personal settings) and FF directory from ProgramFiles as well. Then made a fresh installation of release version. It didn't help :/

Maybe this is stupid but i think the whole thing started to happen when i started to use "Group your tabs" feature. There should not be any connection between font rendering problem and this but who knows maybe there is...

Create a new profile as a test to check if your current profile is causing the problems.

See Basic Troubleshooting: Make a new profile:

• https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Creating new profile didn't help. In the meantime I discovered which font is rendered incorrectly. It is Verdena (see my screen shot: http://img80.imageshack.us/img80/1390/verdenaff.png ) Is there any way to fix it ?

Did you try to remove that not working Verdana font?

You can do a font test to see if you can identify corrupted font(s).

• http://browserspy.dk/fonts-flash.php?detail=1

How to remove this font ? You mean Start->Control Panel->Fonts->remove Verdena ? Or should i remove this font somewhere in FireFox ?

You would have to do that via the Control Panel or move the verdana*.ttf files from the Windows\Fonts folder to another location.