Since the bookmarks item in the new library toolbar button is no replacement for the "old bookmarks toolbar button" at all and also because this button can be enabled within the new library button ("Bookmarking Tools" > "Add Bookmarks Menu to Toolbar") it should follow the Photon style. At the moment it's very different from all the other menus and it looks like it was forgotten. I am sure it was not forgotten (probably time constraints or something like that) but it looks really different. ;)

added screenshots for clarification

Is this really a duplicate of bug 1373961? It's not only about the hover/active style. The bookmark folders does not open inside the panel like the items in other menus, there are different arrows if there are submenus and the old bookmarks menu has icons with colors, the icons of the other menus are all black.

(In reply to Sören Hentzschel from comment #4) > The bookmark folders does not open inside the panel like > the items in other menus, We don't intend to fix this per feedback received about people with many-nested bookmarks folders who claim that the new photon menus make them less efficient. Equally, we have no intention of generally changing the photon menus to use "old style" menus for now. > there are different arrows if there are submenus As a consequence, I don't believe we don't intend to fix this, but if we do, it can happen in bug 1373961. > and the old bookmarks menu has icons with colors, the icons of the other > menus are all black. This is not quite true, the other bookmarks / history / synced tabs menus all use favicons and therefore all use coloured icons, too. If there are icons for the builtin menu that need updating, we can do it in bug 1373961. I'll update the summary. In any case there's no point having several bugs open for a style update to the same widget.