Steps

1. Click Bookmarks > Organize Bookmarks > Click Organize > New Bookmark
2. Type the Name of the Smart Bookmark
3. In Location:

The location field is where you will configure the smart bookmark.
The structure is place:queryType=(0-2)&sort=(0-20)&maxResults=(number of results)&terms=(optional)

queryType: What Firefox will search through to find bookmarks.

0- Searches only your history
1- Searches only your bookmarks
2-Searches both your history and bookmarks

sort: How Firefox sorts the bookmarks it finds.

0- Natural bookmark order
1- Sort by title, A-Z
2- Sort by title, Z-A
3- Sort by visit date, most recent last
4- Sort by visit date, most recent first
5- Sort by url, A-Z
6- Sort by url, Z-A
7- Sort by visit count, ascending
8- Sort by visit count, descending
9- Sort by keyword, A-Z
10- Sort by keyword, Z-A
11- Sort by date added, most recent last
12- Sort by date added, most recent first
13- Sort by last modified date, most recent last
14 – Sort by last modified date, most recent first
17- Sort by tags, ascending
18- Sort by tags, descending
19- Sort by annotation, ascending
20- Sort by annotation, descending

maxResults: The number of results the smart bookmark will show.

terms: Only used if you want to search for specific words in the title of the bookmarks.

domain: Search for bookmarks only in a certain domain.

Examples:
10 Most Visited Bookmarks:
place:queryType=1&sort=8&maxResults=10
10 Most Visited Sites with “Screencastr” in them:
place:queryType=0&sort=8&maxResults=10&terms=screencastr
5 Most Visited screencastr.com Pages:
place:queryType=0&sort=8&maxResults=5&domain=screencastr.com