Why does addon's icon disappears, if `install.rdf` present?

After i’ve install my addon i don’t see the toolbar icon in firefox.
I’ve faced this issue during development. If install.rdf file is present, the addon’s icon is not appears in firefox toolbar. However, it works absolutely fine without install.rdf. What is wrong?

It is impossible for an addon to install into Firefox without an install.rdf (well unless its a WebExtension).

Can you please provide structure, like what folder your icon is in, and what your install.rdf says.