You can do this by simply downloading the standard rapid release version and installing it over top of the Firefox Beta version. This will replace the version Beta version with the standard version without erasing your Firefox data.

NOTE: If you would like to use caution and backup your Firefox data before attempting to replace the Firefox Beta version, you can follow the steps found in the Back up and restore information in Firefox profiles support documentation.

Thanks Wesley!

RDCanfield1962 said

I would rather not be included in the beta version testing, and would like to set firefox to only using the solid complete versions. I cannot find how to change the settings on this anywhere.

A Firefox Release version will not update to a version on another channel including Beta.

A common occurrence I see for people getting on Beta channel without intending to. Say when people decided they want to use another version and grab it from say http://archive.mozilla.org/pub/firefox/releases/ they may not notice the b and a number on end of version. For example 61.0b7 means it was the seventh Beta build of 61.0 and not a Release.