I am not sure about the solution which I am going to mentioned below but you should try and let me know whether it is working or not.
You should locate for the userChrome.css from the home folder.
Now you have to launch the text editor on the system of yours.
Code:
#appmenu-toolbar-button {
list-style-image: url("chrome://branding/content/icon16.png");
}
#appmenu-toolbar-button > .toolbarbutton-text,
#appmenu-toolbar-button > .toolbarbutton-menu-dropmarker {
display: none !important;
}
Now save the same you have to restart the Firefox on the system and see whether it is working or not.
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