Several Firefox 4 users, especially those on Windows, have noticed that some, and sometimes even all, fonts are blurry in the browser. Many users have identified the built-in hardware acceleration as the main culprit for blurry fonts, and noticed that the fonts render fine again if hardware acceleration is disabled.
You need to do the following to disable hardware acceleration. Click the Firefox button, and then Options from the menu. Users who have enabled the old menubar need to click on Tools > Options instead.
Hardware acceleration is an option under Advanced > General in the options window. Uncheck "Use hardware acceleration when available" to disable the feature in Firefox. The changes should be visible after a restart of the web browser.
Disabling hardware acceleration impacts several areas of the browser, not only font rendering. Users may experience slower page rendering or multimedia applications afterwards. Before you do something that drastic, you may want to update the drivers of your graphics adapter first.
Some users were able to correct the font display issues in Firefox after installing the Anti-Aliasing Tuner add-on for the browser. The extension basically offers in depths configuration font rendering options.
With the help of this tool it is possible to tweak the Firefox font rendering settings. The add-on distinguishes between small and large fonts, and offers the same set of customizable options for both.
Available for selection are four anti-aliasing modes, including ClearType, Aliased and Default. The rendering modes that become available depend on the mode that was selected. Among the rendering options are GDI Classic and Natural, Aliased, Natural and Default.
The two remaining options can be used to enhance the contrast by a specified percentage and to select the ClearType level.
The contrast determines the darkness of text, from 0 for no enhancements to 100%. The default setting is set to 50% on most systems.
Similar options are available for the ClearType level. A value of 0 would mean that ClearType is disabled while 100% would mean it is fully enabled.
If you have an eye for detail you probably do not need visual aids to see which font rendering settings display the clearest fonts in Firefox. Everyone else may want to take screenshots, with a 100% quality setting, to compare the different font rendering modes effectively.
It may also help to ask someone standing by to take a look to get a second opinion. Then again, if you have to work on the computer it is probably best if you make the final decision.