Firefox 5.0 is just the next regular security update for 4.0.1 users, scheduled for release on June 21st. However, because of the new release model for Firefox, we also need to update the Firefox translations at the same time (and we will do for all future Firefox security updates). This means that we also need to provide updated language-pack-xx-* packages with the Firefox 5.0 update.
In order to prepare for this, we are splitting Firefox translations out of the main language packs at the same time as the 5.0 release. This means that in the future, we will not need to respin all of the language packs for a Firefox security update (this incurs quite a significant QA penalty). Because of this packaging split and because the new language packs contain updated translations for all of your other applications (exported from Launchpad), they need to undergo extended testing in natty-proposed before we release them to everybody on June 21st. However, testing them also depends on the newly split-out Firefox translation packages existing in natty-proposed (else the package relationships will not be satisfiable for language pack testers).
We took the decision to upload the current beta to natty-proposed because if we wait for the release build before we start testing the language packs in natty-proposed, we will have to delay the next security update. Note that the current beta is actually more of a RC than a beta (this is the last planned beta build before the final release build, and is not likely to change between now and June 21st)
We aren't planning to publish the beta release to all users - it will be removed on June 21st when we publish the final 5.0 release to natty-security and natty-updates. Please remember that natty-proposed is an opt-in repository where we upload packages for testing stable release updates, before we publish them to all users.