hello 1emanresu, this is an icon to share a page through various social networks (it won't work on built-in sites like the default homepage).

in case you have no use for it, you can also remove the button from the toolbar: Customize Firefox controls, buttons and toolbars

This is a new functionality provided by firefox used for sharing the current page with three different services,such as Linkedin,Twitter and Pocket .

In this way linkedin "Manage your professional identity. Build and engage with your professional network. Access knowledge, insights and opportunities." While Firefox twitter "Share to quickly author tweets about the pages you discover across the web. They also shorten URLs for quick sharing using the t.co link shortener." and Pocket "services The best way to save articles, videos and more"

This is all new services provided by Firefox for keep the web experience rich enabled.