Answer

Yes. Tirthankars become Siddhas after attaining moksha.

While they are living after attaining Keval Gyan, they are called Arihants or Kevalis. After they shed all remaining karmas and leave the body at the end of their life, they attain Siddha state.

Explanation

A Tirthankar’s spiritual journey can be understood like this:

• Before Keval Gyan: They are highly advanced spiritual souls progressing toward liberation.
• After Keval Gyan while living: They are Arihants — perfect beings who have destroyed the destructive karmas and teach the path to liberation.
• After nirvana/moksha: They become Siddhas — liberated souls without body, karma, birth, death, or worldly existence.

So, all Tirthankars are Siddhas now, because all 24 Tirthankars of the present time cycle have attained nirvana.

Spiritual Understanding

In Jain Dharma:

• Arihant means a liberated-in-life omniscient being who still has a body.
• Siddha means a fully liberated soul beyond the body and beyond all karmas.
• Tirthankar is a special Arihant who establishes the fourfold Jain Sangh and shows the path across the ocean of worldly existence.

Takeaway

A Tirthankar is an Arihant while living after Keval Gyan, and becomes a Siddha after moksha.