Why does MEGAsync need my password? I have downloaded the MEGAsync app (because MEGA won't let me download in Firefox), and whenever I launch it, I get a password prompt with no stated reason:

Why does MEGAsync need my password?
MEGAsync is a desktop app made by MEGA, which uploads and downloads files to and from MEGA's cloud platform. I am trying to use it to download a large file, because MEGA won't let you download large files in Firefox.
macOS Sierra
MEGASync requires an admin password because it installs a Finder plugin.