Re: How do I restore a folder that I deleted full of important bookmarks?
If you use the Time Machine then you can try to restore a copy of places.sqlite from just before this deletion.
If you have accidentally removed bookmarks then use "Organize > Undo" in the bookmarks manager (Bookmarks > Show All Bookmarks) to get them back.
The Organize button is the first of the three buttons on the toolbar in the Library (Bookmarks Manager).
That only works if you haven't closed Firefox.
If it didn't work then you can restore the bookmarks from a JSON backup.
This will replace all current bookmarks and you lose bookmarks that were added since that backup was created.