You might want to try out the new beta with Direct2D and DirectWrite GPU hardware acceleration turned on.
You can download Firefox 4.0 Beta 4 here.
Changes in Firefox 4.0 Beta 4:
* Firefox 4 Beta 4 is available in 39 languages.
* Firefox Sync is now included by default.
* Users can organize their tabs using Firefox Overview.
* An experimental API is included to provide more efficient Javascript animations.
* Firefox now supports the HTML5 video "buffered" property.