All perfect praise be to Allah, the Lord of the worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the intended meaning of mountainous areas is these areas in which there are a lot of mountains, then if they are in a high place so that they can see the sunset, the matter is clear. If they are in a low place in which it is difficult to see it, then the residents deduce the beginning of the time of the Maghrib prayer and the time of breaking the fast by the coming of darkness from the direction of the east and not seeing anything of the rays of the sun on walls and the summits of mountains.
An-Nawawi said in Al-Majmu‘:
"The beginning of the time of the Maghrib prayer is when the sun sets completely. There is no difference among the scholars concerning this. Ibn Al-Munthir and countless people said that there is a consensus about this. The followers of our school said, “What is considered is the fall of the sun completely. This is clear in deserts. Sheikh Abu Hamid and his companions said, ‘After complete sunset, the remaining rays are not to be considered. Rather, its (Maghrib) time starts in spite of the remaining rays. In urban places and summits of mountains, what is considered is that they see nothing of its rays on walls and the summits of mountains, and that the darkness comes from the east.’”
The Maliki scholar Al-Hattab said in Mawahib Al-Jalil, “Ibn Bashir said, ‘The time of the Maghrib is when the sun disappears in a place where there are no mountains. As for a place in which it sets behind mountains, one should look at the direction of the east; if there is darkness, this indicates the sunset.’”
We have not come across a Fatwa of Sheikh Ibn Baz about this matter.
Allah knows best.