Firefox 4 is the latest stable edition to the latest Firefox family. With Firefox 5 and Firefox 6 (aurora) is still on the development table, Firefox user now have the the feature called hardware acceleration which designed for faster performance just like other browser such as Chrome which is set for HTML5 and CSS3 compatibility.
One of the major set back for Firefox 4 user with hardware accelerator capability is the rendered font may be a little bit eye sore. disabling the hardware acceleration in Firefox 4 may fix the problem.
How to Disable Hardware Acceleration in Firefox 4
1. Open Firefox, click on the orange Firefox button on the upper left and select “Options”.
2. Go to “Advanced” tab and untick “Use hardware acceleration when available” option.
3. Click on OK and restart your browser.
Another way to fix the issue on your Firefox 4 browser is simply update the display driver on your computer or you can use this plugin for Firefox.