hello gmarshman, firefox 28 is currently the latest released version of the browser so that notice appears right. when your other computer is on firefox 29 already, it likely uses the beta channel, which you could install on the second pc as well: https://www.mozilla.org/beta

The computer running version 29.0 is running the (Beta) version of Firefox, whereas your computer running 28.0 is running the release version.

Version 28.0 is the most recent released version, but other versions can be downloaded for testing purposes from the Firefox channels website.

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edit: Beta is not a Nightly (removed word by Beta to reduce confusion) and the unstable Nightly channel is not a good idea for regular users to use so link was changed

Firefox 29 (beta) has the new 1.5 version of Sync, where Firefox 28 (earlier) uses the previous version of Sync. Once a Sync account has been upgraded to the new 1.5 version of Sync it won't connect with a pre-29 version of Firefox. So you are looking at a 6 week period of incompatibility, until your release version is updated to Firefox 29.

On a positive note, the new version has a greatly simplified (for the user) method of adding new devices to an existing Sync account and for re-connecting a device that was "un-linked" from the Sync server. The 3rd authentication "key" - using the Sync Key - has been eliminated. The data encryption "key" will be created (and "compared" at time of login) using the account / user name and the account password; the 3rd key won't be needed by the user.