I transferred my Firefox bookmarks to a new computer and although the bookmarks are visible, when I try to save a new bookmark it doesn't show any of my subject folders in the drop down destination list.

Look at this [[Bookmarks#w_making-new-bookmarks | paragraph 2]] you will get a similar dialogue box. Note the two down arrows, one displays the last few options you have used, * use '''choose''' if you do not see the folder required * or '''use the arrow to the right instead''' which displays all your folders.