In the nightly builds of Firefox that will form the basis of the August 23 release of Firefox 104, an edit mode has been added to the built-in interface for viewing PDF documents, which offers features such as drawing custom labels and attaching comments. The pdfjs.annotationEditorMode parameter is provided on the about:config page to enable the new mode. Until now, Firefox's built-in editing capabilities have been limited to supporting the interactive XFA forms commonly used in electronic forms.
After activating the editing mode, two buttons will appear in the toolbar - for attaching text and graphic (freehand line drawings) labels. Color, line thickness and font size are configured through the menus associated with the buttons. When you right-click, a context menu is displayed that allows you to select, copy, paste and cut elements, as well as undo the changes made (Undo/Redo).
Source: opennet.ru