This question as it is written is not related to (Python) programming, but to the specification of the .desktop file. Citing specifications.freedesktop.org/desktop-entry/latest/…: Icon: Icon to display in file manager, menus, etc. If the name is an absolute path, the given file will be used. If the name is not an absolute path, the algorithm described in the Icon Theme Specification will be used to locate the icon. See also specifications.freedesktop.org/icon-theme/latest/#icon_lookup
.desktopfile. Citing specifications.freedesktop.org/desktop-entry/latest/…: Icon: Icon to display in file manager, menus, etc. If the name is an absolute path, the given file will be used. If the name is not an absolute path, the algorithm described in the Icon Theme Specification will be used to locate the icon. See also specifications.freedesktop.org/icon-theme/latest/#icon_lookupfind /usr/share/icons -name 'firefox*'There are different files depending on theme and requested size.