Short answer from the Jain standpoint:

• Jain Dharma is anādi–anant (beginningless and endless). So, in our own tradition, asking “which came first?” does not really apply—Jain Dharma has no starting point in time. It is eternally re‑taught in every time‑cycle by the Tirthankars. In the present avasarpiṇī (descending half‑cycle), Ādināth (R̥ṣabhadeva) is the first Tirthankar, followed later by Mahāvīr Bhagwān as the 24th.

Historical note (for context):

• Modern historians work with available evidence and dates. They usually place Bhagwān Mahāvīr in the 6th century BCE and accept that Jain tradition speaks of many earlier Tirthankars. Vedic/Hindu traditions also go very far back. But the Jain religious view remains that our Dharma is timeless and periodically revealed.

If you wish to read more in simple language:

• Antiquity of Jain Dharma
• How Jainism understands “How old is Jain Dharma?”
• Ādināth (R̥ṣabhadeva), the first Tirthankar of this time‑cycle