Currently, there is no native method to edit or add page bookmarks to a PDF document.
For people that use the integrated PDF viewer of Firefox, it would be very helpful to be able to add pages to outline or to add selected text to outline (especially for PDFs that don't already have an outline) for staying organised in large PDFs.
Another feature that complements this is to control the outline level (level 1, level 2, etc.) for added elements, or even for an existing outline, by right-clicking on an element in the PDF outline and selecting a wanted change in a menu.