Have you checked out those bookmarks? Did you create them? Many websites display an option to bookmark their site. Whether a bookmark can be placed without asking the user I don't know.

Spam is a term not normally associated with bookmarks. Most common usage of that term with regards to personal computers pertains to "junk mail" or unsolicited email; and often there is an element of Malware involved, that can affect a device when a hyperlink is clicked or a malicious attachment is opened.

The folder labeled Other Bookmarks was changed from Unsorted Bookmarks in Firefox 48. But depending upon how old a set of bookmarks is or if a bookmarks.html file from another browser has been merged in the past, the Other bookmarks folder may be a separate folder. I had that happen with one of my Profiles that I have been using for like 10 years now.

That change happened at the same time as Recently Bookmarked was moved out of a Blue "Smart Folder" into an inline listing in the two drop-down Bookmarks menus; doesn't appear in the Bookmarks Sidebar.

Bookmarks that aren't saved to the Bookmarks Toolbar or saved to a specific folder will be in the Other Bookmarks folder, as they did previously in the Unsorted Bookmarks with Firefox 3.0 up thru Firefox 47.0. Single click on the Star is what saves a bookmark to that folder; whereas a double-click will open the dialog box that allows the new bookmark to be saved to a specific folder the user chooses.

I doubt if a bookmark can be "saved" by merely viewing a webpage; IMO, some type of "click" on something by the user would be needed. As Fred mentioned, a webpage can have a hyperlink for Bookmark this Page that allows the user to save that page as a new bookmark; but typically that script also selects the Properties preference for that new bookmark for Load this bookmark in the sidebar. So it's best not to use those types of hyperlinks that are on some webpages.