Unfortunately the above post seems to address the "fire of God" rather than the question presented in the opening post, which was:
How do we understand what the Bible means when the word "forever", "eternal", "for ever" is used in Scripture in reference to something or someone ?
It would seem that various bible writers had different intentions when using the terms. For example, Jonah 2:6 vs. John 3:16. The same term did not always mean exactly the same thing to all writers or hearers. Each use should probably be examined on a case by case basis, especially if the passage is being discussed, or used to support some doctrine or idea.