These words were used in a chat, according to jisho.org both terms mean being relaxed or taking it easy. How do you use them?
They belong to different word classes. のんびり is an adverb and suru-verb, and くつろぐ is a verb on its own. That means のんびり (without する) and くつろぐ are never interchangeable.
The difference between のんびりする and くつろぐ is small and they are mostly interchangeable. But くつろぐ tends to refer to relaxing physically (e.g., in a sofa, doing almost nothing). のんびりする tends to refer to mentally feeling relaxed, not being pressed for time. You can say のんびりする even when mild physical activities are involved (e.g., ハワイでダイビングやドライブをしながらのんびり過ごした).