Open the Bookmarks Manager; Press the Alt or F10 key to bring up the toolbar, and select Bookmarks. Hot key is <Control> (Mac=<Command>) <Shift> B. Firefox 84+ <Control> (Mac=<Command>) <Shift> O.

You can add the "Bookmarks Menu" button that shows the bookmarks in a drop-down list to the Navigation Toolbar and that has a button to open the library. You can drag the Bookmarks Menu button from the Customize palette to the Navigation Toolbar

• https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

See also "How do I add the Bookmarks Menu button to the toolbar?":

• https://support.mozilla.org/en-US/kb/bookmarks-firefox