I just copied the application firefox from
/usr/share/applications to my home directory and then it shows me a
.desktop file. When I try to open it, it says "Untrusted application launcher". How can I open it, and what is it? What does it do?
According to https://wiki.archlinux.org/index.php/desktop_entries:
The freedesktop Desktop Entry specification provides a standard for applications to integrate into a desktop environment. Desktop entries are the configuration files that describe how an application is launched and which data it can handle. They also configure how an application appears in a menu with an icon, which is subject to the related menu specification standard.
The most common desktop entries are the .desktop and .directory files. This article explains briefly how to create useful and standard compliant desktop entries. It is mainly intended for package contributors and maintainers, but may also be useful for software developers and others.