Like GMailThis or BlogThis bookmarklets, the Blog This extension adds a button to Firefox that would launch Windows Live Writer prepopulated with content, title and URL of the current web page.
This extension can be used to either blog the whole webpage, or just selected snippets.
Download Blog This for Firefox 2
Download Blog This for Internet Explorer 7
Related Tip: You can add any custom HTML code for Blog This - Select Tools->Preferences->Blog This. Now select Link, Image or Snippet from the Content box and click "Customize HTML". [see screenshot]
Source: Windows Writer Team Blog