Firefox's dev and beta releases are automatically updated when new releases are made by the Firefox project. However, as is the nature of betas, sometimes this Firefox beta breaks compatibility with Selenium, making it impossible for Sauce Labs to use. Because Firefox and Selenium are both Open Source projects, not under the control of Sauce Labs, we're unable to set project direction and ensure this functionality never breaks.
End users rarely install Firefox beta or dev. Those that do tend to understand that functionality may be missing or broken. In addition, the beta & dev releases are made available mostly to assist with testing the Selenium functionality of said releases.
Sauce Labs makes very minimal changes to the browsers we offer. We also take care to offer exactly what we say we do. Making custom patches to Firefox or Selenium would make them no longer reflect reality. If Sauce Labs can't spin up Firefox beta or dev with Selenium, it's likely that this functionality doesn't work locally on all platforms. We also don't want to offer a beta or dev release that isn't the same as the Firefox project's current beta or dev release, because that would just lead to confusion.
We recommend customers testing commercial web applications for end user consumption stick to released versions of browsers. These have passed testing by their authors, the Selenium project and Sauce Labs, and are known to have, at the very least, passed Selenium and Sauce Labs' test suites.