"Download History Cleaner" is the easiest way to clear your browser download history for a specified period. You can define the custom period on the options page. Also, you can define the area (zone) in which you want the data to be cleared.
Using this extension it is possible to delete the download history of three different zones: 1. Normal websites, 2. Protected websites (not supported in Firefox yet), 3. Extension zone (not supported in Firefox yet)
Definitions: -> Extensions zone: Extensions and packaged applications a user has installed (not yet supported in Firefox) -> Protected websites zone: Websites that have been installed as hosted applications (not yet supported in Firefox)
