You can try

Open Firefox > Click Firefox Menu button > Select Bookmarks > Show all bookmarks.

You accidentally deleted the bookmarks right? Try by clicking on Organize button and select Undo or keyboard ("Ctrl + Z").

The Firefox is automatically creates backup of bookmarks regularly actually, if you deleted any bookmark few days back, you are able to restore it. Click on Import & Backup > Select Restore After that, it will show you all the backups created and you can select the bookmark file you want.

Was this a so called smart folder button on the Bookmarks Toolbar that shows the most visited websites in a drop-down list used in older Firefox versions ? Smart folders are no longer added by default in current Firefox releases, so you may have to re-add such a bookmark if you lost it.

If you haven't closed Firefox yet then you can try "Organize -> Undo" (Ctrl+Z) in the Library like posted above. You may have to use Undo more than once (Firefox keeps a full undo stack of changes to bookmarks for the current session).

You can create a new bookmark and paste this place URI in its location field if you want to create a smart folder that shows the result of a places.sqlite database query. The maxResults parameter sets the number of results to show. You need to do a Cut/Paste to another folder or close and restart Firefox to make this smart folder work properly (it will get a cogwheel icon).

Name: Most Visited
Location: place:sort=8&maxResults=10

• https://developer.mozilla.org/en-US/Mozilla/Tech/Places/Places_query_URIs