Here is the full list of all 10 Firewatch achievements worth 1,000 gamerscore. It takes around 4-5 hours to unlock all of the achievements on Xbox One.
Completed Day 2
Adopted a turtle as a pet. The average lifespan of a box turtle is fifty years. It will outlive you.
Completed Day 1
Complete Day 76
Complete Day 77
Complete Day 79
Fill a disposable camera with photos.
Have a hands-on encounter with the local wildlife.
Find a bee.
Find and hear the recording of the song "Ol' Shoshone."