I haven't played with the latest FF4 beta, but FF4b7 it was pretty simple to add a Feed button, as it's provided, though defaulted to off. The one issue with it is that it isn't contained in the awesome bar any longer.
If you want to re-enable it, you just have to configure your toolbars, and find the feed icon, then position it where you want it. Sorry, this is non-descript, I don't currently have access to Firefox so that I could provide the complete steps.
If you want to re-enable it, you just have to configure your toolbars, and find the feed icon, then position it where you want it. Sorry, this is non-descript, I don't currently have access to Firefox so that I could provide the complete steps.