It's a Firefox thing. The context menu in pwa mode is different from the normal one. This is most likely a bug.
A workaround I sometimes use, is to go to the notification drawer, click the silent pwa notification to copy the URL of the current page, share the copied URL with firefox and finally use the normal context menu.
No there is no way to customize the menu. I found an old bug that mentions adding a share option to the context menu. It is probably worth it to either kick this old bug, mentioning what is still missing, or to create a new bug describing the current issue.