The star will turn blue and will be added to your bookmarks. You can also press Ctrl/⌘ Cmd+D. There are a few different changes you can make to the bookmark from this pop-out window:  You can change the name of the bookmark, which is the same as the page's title by default. You can select a folder for the bookmark by choosing one from the drop-down menu. You can create a new folder by clicking the ∨ button and then clicking New Folder. Add tags to the bookmark. Firefox allows you to tag bookmarks with words or short phrases. You can then find your bookmarks later by using these tags. Click the ∨ button to view all of your tags. The bookmarks bar is a way to quickly access your most important bookmarks. It appears beneath the address bar. Click the Clipboard button next to the Search bar, select "Bookmarks Toolbar", and then click "View Bookmarks Toolbar" to toggle it on or off. If your bookmarks bar is showing, you can drag a websites icon directly onto the bar to quickly add it as a bookmark. You can open the Library window to manage all of your bookmarks. Click the Clipboard and select "Show All Bookmarks" to open the Library window to the All Bookmarks section.  Click and drag bookmarks to move them. You can select multiple bookmarks by holding Ctrl (Windows) or ⌘ Command (Mac) and clicking each one. Right-click and select "New folder" to create a new folder for your bookmarks. The folder will be created in your current location. Delete bookmarks by right-clicking on them and selecting "Delete". You can delete multiple bookmarks at once if you have them all selected.

Summary:
Open the page you want to bookmark. Click the ☆ button next to the Search bar. Click the ★ to open the bookmark's details. Toggle the bookmarks bar. Manage your bookmarks.