Why are my bookmarks showing up in my show history?
Most visited is more like history than most visited. Some seldom used bookmarks are populated in show history. I can delete them but they reappear.
Alla svar (1)
Could you explain in more detail where you are looking when you see things mixed up?
Firefox's Library dialog
The Library lists Bookmarks, History, and Downloads -- you'll see a tree diagram in the left column for which category you want to look in. You can open this dialog from any of those perspectives: "Show All Bookmarks" (Ctrl+Shift+b), "Show All History" (Ctrl+Shift+h), or "Show All Downloads" (Ctrl+j). But they should be distinct and not mixed up...
Most Visited Smart Folder
The Most Visited folder -- should have a special blue folder icon with a magnifying glass icon -- is a list generated by a search of your history. I don't think a site being bookmarked or not influences whether it appears on the list.
If you want to cross-check the data, you could use the Library dialog to view the Visit Count. In the left column click History, and in the search box at the upper right enter http to find all addresses starting with http or https (nearly everything). If your Firefox doesn't display the Visit Count column, you can add that to the list by right-clicking an existing column heading and checking the box for Visit Count. Then you can click the Visit Count heading to sort on that column. One click for ascending sort (low to high) and a second click for descending sort (high to low). High to low should match the list under Most Visited, assuming all is working correctly.