For whatever reason, that option/selection is only available when the Bookmarks menu is set to appear under the main Firefox menu. If you have Bookmarks as a toolbar button, the option will go missing.

I don't know of a way to add the option back in via the UI; I would suggest using the right-click menu as a minimum-hassle workaround (right-click on a page, select the "Add Bookmark" icon).

PS. If you'd like to let the devs know your opinion about how the feature is implemented, you can leave feedback at https://input.mozilla.org/en-GB/feedback.

Do you mean the toolbar button that looks something like a clipboard, or the Bookmarks menu on the classic menu bar?

The triangles respond to being hovered over. They change from black to gray when you've reached the end of the menu in that direction.

As noted in the first reply, you can bookmark the current page in other ways, which may or may not be more convenient:

• Ctrl+d
• right-click a blank area of the page > star icon
• drag the "circled i" icon from the left end of the address bar to a spot on the bookmarks toolbar or bookmarks menu

• There is a star like button next to the search bar on the Navigation Toolbar to bookmark the current web page and a "Show your bookmarks" button next to it to open the Bookmarks in a drop down menu.
• You can find "Show All Bookmarks" to open the Bookmarks Manager (Library) at near the top of the drop-down list.
• If the current page is bookmarked then "Bookmark This Page" in the Bookmarks menu as well as the tooltip of the star changes to "Edit This Bookmark".

cor-el said

• There is a star like button next to the search bar on the Navigation Toolbar to bookmark the current web page and a "Show your bookmarks" button next to it to open the Bookmarks in a drop down menu.

I have that button hidden so I forget it's there, thanks for the heads-up cor-el!