Why chromium is explained in the article (better support for many apps) but you can always change it to firefox, also detailed in the upgrade article.
I upgraded from 38 to 39 with no issues. It’s honestly not hard or dangerous, just copy paste a few lines then paste the last ones to prompt the upgrade. The only catch is that if you had KDE already you won’t need to remove some of the packages it tells you about. Don’t overthink it imo.