That folder will reappear automatically once there is history content from the previous day(s).

You can't recover deleted history, so you will have to revisit sites again to create new history entries.

Cor-el , Thanks for feedback on this issue. These folders have not reappeared as you indicated. In fact, if i go to History tab on Mozilla, then to tab for "show all history" a sidebar drop down of my bookmarks folders opens on left of pg.

First 3 folders under History are:1 This month 2. September. 3. August .
The 7 day history folder is now not there either, or the Yesterday  or Today Folder as previously reported.
Further on the right of this page that shows browsing history,  the last date of recorded browsing history (In "This Month" folder) goes back to Oct. 5th, 2010. Apparently the date in which I accidentally deleted the default folders named "Today" & "Yesterday".  Any additional suggestions would be greatly appreciated.