Notes Browser Plug-in is a product for accessing Notes/Domino based applications inside your browser, and not require any modification to your applications.  For accessing Mail inside a browser, we recommend IBM iNotes (Previously known as Domino Web Access - DWA).  We have modified Notes Browser Plug-in to form a mailfile URL in http when the user tries to double-click on the mailfile icon on the workspace or the bookmarks.  For example, if my mailfile was on a server xyz/ibm, we find out it's http name (somethinglike xyz.ibm.com) and then append the pathname to the mailfile to it and fire it up in the browser.  So it could come up as:

https://xyz.ibm.com\mail\abc.nsf

Notice that we always try to use https.  But if your Domino Server's URL is 'http' only, where you want the URL launch your mailfile to be :

http://xyz.ibm.com\mail\abc.nsf

then simply add the following to your notes.ini file:

BrowserUseHttpForiNotes=1

.