On the page you want to bookmark, do a right-click in an empty area of the page. At the top of the menu, press the Star.
Hot key; <Control> (Mac=<Command>) D.

Or, Press the bookmarks Star on the toolbar.

This will place a bookmark in the Unsorted or Other Bookmarks Folder. To edit or move that bookmark, press the Star again.

A window will come up with the name of the page and its web address.
You can edit these entries.

Next, select what bookmarks folder you want this bookmark in.
If you do not see that folder, go to the far right of the Folder
line, and press the down arrow. You can also create a new folder.

You should be able to drag the tab to the Bookmarks Toolbar or to a folder on this toolbar to create a bookmark.

See also this extension:

• https://addons.mozilla.org/firefox/addon/default-bookmark-folder/