I want to downgrade to Firefox X stable from Y beta due to add-on incompatibilities.
For example from Firefox 11 beta to Firefox 10 stable.
How can I go to stable channel?
This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.
If you use the latest version of Ubuntu uninstall Firefox 11 beta and download Firefox 10 with the Internet (from any stable site), but if you are using an older version of Ubuntu you use menu to return application to previous version (using Synaptic, of course).
If you are using the beta channel PPA,you could just remove the PPA from "Software Sources".