I haven’t been paying the new beta’s of Firefox 4 too much attention (except to note that beta 5 has hardware acceleration turned on by default) as I’ve become a solid Chrome user, but I have noticed some minor differences in how it renders pages when compared to the current stable release of 3.6.9.
One simple example of this is the layout of the “subscribe buttons” in the sidebar of this site.
As you can see from the image below the buttons are where they are supposed to be in 3.6.9 but spill over in beta 5.
With the exception of Firefox 4 Beta 5 the sidebar displays properly in all other browsers. I’ve checked this against IE8, Chrome, Opera, Firefox 3.6.9 and Firefox 4 Beta 5 on both XP and Windows 7.
I hope this small issue is not indicative of larger issues ahead in terms of layouts and rendering in Firefox 4.