The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Which font is that? Is there a demo page online for viewing/tweaking?

MDN reference: https://developer.mozilla.org/en-US/docs/CSS/font-weight

See also http://people.mozilla.org/~jdaggett/tests/arialblackweights.html

@tyler: Sorry I did not mention that I already reset Firefox. @jscher2000: The font is the famous Source Sans Pro. Of course the rendering is correctly on http://www.google.com/webfonts/specimen/Source+Sans+Pro

Here is a demo page http://test78364.test-account.com/_/

Any clues?

Looks fine here on Linux with the Source Sans Pro font.

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

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 disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

• https://hacks.mozilla.org/2010/09/hardware-acceleration/

Try to set the gfx.content.azure.enabled pref to false or if this didn't help disable Direct2D by setting the gfx.direct2d.disabled pref to true on the about:config page and leave hardware acceleration otherwise enabled.

• http://kb.mozillazine.org/about:config

Hi dreerr, at first I got the same results you got, but then I noticed that the font download was being blocked by NoScript, so another font was being substituted. After I allowed the font download, Firefox lined up with IE9 and Chrome (on Windows 7 Pro 64-bit).

Odd enough this problem only seems to exist under OS X and then also under certain circumstances. In BrowserLab everything also seems fine to me. I will do a complete reinstall of FF on my system and see if the problem persists. Thanks for all the answers!

I have had the same issue.

I determined it's due to KB2670838 update that made changes to the following files.

   D3D10SDKLayers.dll
   D3D11SDKLayers.dll
   D3D10ref.dll
   D2D1debug.dll

Platform Update for Windows 7 x64-Edition (KB2670838)

http://support.microsoft.com/kb/2670838

DO NOT INSTALL THIS UPDATE AT THIS TIME

Hi HiroPro, just FYI, the original poster is running a Mac, but your post may be useful to Windows users reading this thread who have the same symptoms, or Mac users with the same graphics chips.

Note: First attached image

As a Firefox user, we shouldn't have to go through settings to try and resolve this problem, as it won't work for a wide variety of users anyway.

Any news on Firefox resolving this? Has been annoying me for years.

See my attach image, you can see the differences.

Note: Second attached image

I like how they use "Open Sans Light" on there website, as Helvetica with -webkit-font-smoothing would give the same visual look in terms of weight.

Quick comparison between the live site in Firefox (Left) using "Open Sans Light" vs a screenshot of Safari (Right) with the font "Helvetica" with font smoothing on.

Actually this is not a problem with Firefox, it is happening because Webkit based browsers have support for -webkit-font-smoothing property which allows a site owner to alternate the system font smoothing on Mac OS X (which I think is completely wrong). Firefox on the other hand always uses system wide settings for font smoothing.

In OP's case there is -webkit-font-smoothing: antialiased; somewhere in CSS which disables the native system wide antialiasing in Webkit browsers, while Firefox is using system settings.

The right solution is to remove -webkit-font-smoothing: antialiased; from your CSS and reduce the font-weight a little bit

Yes, but -moz should create a similar prefix, as icon fonts just don't cut the mustard on the internet in this day and age.

The right solution is to serve beautiful websites to people that have a browser that cares about typography ( -webkit browser. )

Not really a problem for the Window's users, as the fonts render bad across the entire OS. I think they have finally improved this for Windows 7.

-webkit-font-smoothing for life, please give us -moz-font-smoothing.

PS: Digital typography looks better on a Mac. ( Apart from whatever Firefox does. )

If Internet Explorer can render fonts correctly on Windows 7, then Firefox can do the same across both operating systems.

Left to Right

Firefox ( Mac ) , Chrome ( Mac ) , Internet Explorer ( PC ) , Firefox ( PC )