Click “hamburger” in the far right corner of the main toolbar to open the menu, and then click “configure” at the bottom. The icon you are looking for is listed along with other “advanced tools and features” and can be dragged to the toolbar.
I had to restart firefox after that to get it to actually activate the download function when I clicked on it.
Ps. It also took me a while to find the right source for the livereload plugin that actually loads in firefox: (addons.mozilla.org/en-US/firefox/addon/livereload/?src=search), but you seem to be this part coming out.