Thanks for posting your question in the Support Community. Click here for a landing page about AOS firmware releases, notification signup and links to independent release histories by model. Those go back only so far, currently, and I'm not sure you'll see versions before R10.X. Releases that begin with a number instead of "R" are quite old. Click here to search by product and view a more exhaustive list.
At the time of this writing, R11.X releases are the latest versions which support the newest features. However, it is generally recommended to use the most recent long-term maintenance release for production systems unless you need a specific feature in a higher release number. This will ensure the most stability. Right now, R10.9 is the long-term maintenance version and R10.9.4 is the latest release in that series.
Note that there are sometimes new commands or changes to the syntax of configuration commands. New releases can adapt existing configurations, but you should be careful if you ever need to downgrade as certain lines in your config may need to be manually adapted. If there is any question in a downgrade scenario, I recommend opening a case with ADTRAN Support at 888-4-ADTRAN.