Across various Ubuntu releases, Mozilla Firefox official stable PPA has delivered up-to-date Firefox version mainly for Ubuntu 10.04 LTS and 10.10 (recent Ubuntu versions received stable Firefox versions both as an update or via Mozilla Security PPA).
Due to the imminent release of Firefox 10 later next week that is to replace Firefox 3.6.x (on Ubuntu 10.04 LTS), the Firefox stable PPA will be abandoned, furthermore "starting on January 17, Ubuntu 10.04 LTS and Ubuntu 10.10 users will be migrated to the latest Firefox version, and will track the rapid releases going forward, as is currently done in newer releases of Ubuntu. This will enable users to get the numerous improvements offered by new Firefox versions".
All Ubuntu releases will receive the new Firefox versions via the update manager, thus making the usage of PPAs obsolete.
Ubuntu's Chris Coulson recommends the removal of the Firefox stable PPA.