Downgrading should keep the profile and current information stored on your browser. If your history is turned on and to not clear upon closing, you can still fetch what website you were on through your history. The bookmarks you've made should still stay where they are regardless of a upgrade or downgrade.

Note: there may be stability issues due to the fact Beta is not the official released version but developed for testing and development to be the next version of Firefox. You can help contribute to Mozilla by sending in these crash reports so they can triage and fix the crashing and instability.

Andrew, thank you for answering my previous question. I have a continuation. How can I get off the Beta upgrade track the next time a full/non-beta version becomes available?

You can use the current released version, which is 40.0.2 from getfirefox.com

Doesn't that though still mean I'm flagged for being on the Beta track?

What do you mean? If you want to use the current version of Firefox, you would no longer be using Beta. The current version is always a version behind Beta and Beta will always be a version ahead.

I'm either in the "Beta Program" or "Beta user group"

You've asked for assistance regarding downgrading and I've provided an answer. Now, you're asking if you will no longer be part of the Beta testing group, the answer to that would be yes, since you would no longer be using Beta. Alternatively, you can using Aurora or Nightly versions of Firefox (version 42/43) if you would still like to test and contribute to Mozilla.