LetGo is a Firefox extension, can add a 30-second delay when loading certain time-wasting websites. The delay is per-tab. Once you wait on a tab, you can browse time-wasting websites in that tab without obstruction. Browsing to such sites in other tabs will incur a similar delay.
The list of time-wasting websites is stored as a pref in about:config called LetGo.filteredSites.
Example: if you wish to filter nytimes.com, you would follow these steps:
1. Type about:config in your address bar
2. Right click to bring up the context menu, and select New->String
3. For the preference name, enter: LetGo.filteredSites.
4. For the value, enter the address
5. Restart the add-on by disabling it and re-enabling it.

Requirements:
* Firefox

The license of this software is Free, you can free download and free use this browser plug-ins software.