Thanks! Using nightly builds, I am used to running with checkCompatibility "classic"... but I missed the switch to the current version-based usage.
For anyone else [that may wonder exactly what to use as your new pref name], "extensions.checkCompatibility.2.1b" does the trick.
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b12pre) Gecko/20110205 Firefox/4.0b12pre SeaMonkey/2.1b3pre