Griffiths Translation: For, overthrowing what hath ne’er been shaken, thou goest forth alone destroying Vṛtras. For him who followeth thy Law the mountains and heaven and earth stand as if firmly stablished.
Does Rig Veda 3.30.4 say the Earth is immovable?
No, unshakable is the right word instead of immovable.
In the beginning the Earth was shaking or swinging and it's Indra (in some texts) or Vishnu (in some texts) who made Earth unshakable by installing mountains or dividing the Earth in Continents. And today Earth is unshakable, only earthquake occurs sometimes.
Even after the earth was brought up from the seabed and was established above the ocean, it was still swinging like a boat. Narayana then created the mountains to stop the earth from swinging. But the mountains were burnt down by Agni (fire) named Samvartak. The mountains submerged into the ocean. The mountains after having displaced the water had become fixed at their respective places. Therefore, Narayana divided the whole earth into seven islands and created the four lokas just like before. (Source Natural and Flawed Creations chapter of Markandeya Purana)
The below Hindi translation of Rig Veda 3.30.4 seems to be more correct: