You could make a simple Folder Action with the Automator app to do this.
Create a new folder named "Add to Photos". Open Automator and pick "Folder Action". Click at the top where it says "Choose Folder", select "Other…" and select the new folder. In the search field, next to "Actions" and "Variables", type "photos". Drag "Import Files into Photos" to the workflow area. Save and you're done.
Now, if you save an image from Firefox, or anywhere, to that folder it will import into Photos.
You can make specific folders import into specific albums, if you wish, with "Add to Album".
You can put the folders on the sidebar of your Finder windows for quick access in the "Save As" dialog boxes.