Usually new Firefox version available within a few hours of Mozilla's announcement, sometimes faster, depending on the archive builders, and the speed of your local mirror, etc.
You can update Firefox with latest version anytime if you do not want to wait till it available in official repository.
From Lucid (10.04) and higher, Firefox will be upstreamed in main.
You don't need to add any PPA to get the newest Firefox.
Edit: Also, if you could elaborate as to which sites this is an issue on, that would be helpful.