Does LTS releases include repo exceptions like Firefox?

All supported versions of Ubuntu now receive updated versions of Firefox as soon as they are released by Mozilla, there's no need for a PPA or anything like that:

• How do I install the latest stable version of Firefox?

No. LTS is about stability and not the latest and greatest software. That said, as soon as Firefox 3.6 reaches EoL, it will, most likely, be upgraded on all supported releases. The same principle applies to the regular releases, for example, Firefox 4.01 is EoL, and consequently got upgraded in Natty.

Edit: Firefox 9 is now available from the repositories for both Lucid and Maverick, in line with the earlier decision to migrate the users of those releases to the current version of Firefox. https://wiki.ubuntu.com/LucidLynx/ReleaseNotes/FirefoxRapidReleaseMigration

If you enable ubuntu-proposed updates, you will get the firefox 9.0.1 from there. No need for ppa repo. (I use lucid)