In our present time-cycle (this Avasarpini), Bhagwan Adinath (Rishabhdev) is the first Tirthankar.

So:

• There is no Tirthankar “before” Adinath in the current set of 24 Tirthankars.
• The list of 24 Tirthankars in this era actually starts with:

1. Adinath (Rishabhdev) 2. Ajitnath 3. Sambhavnath 4. … and so on, up to Mahavir Swami as the 24th.

In Jain time philosophy, there are infinite past time-cycles with their own sets of Tirthankars, but for our current chovvisi (the 24 that Jains usually refer to), no Tirthankar comes before Adinath.