Antonio, this is some info around your subject:

-1*command lines firefox (not sure all work-there is also Thunderbird options)
to test: run (shellexecute ) : firefox.exe .....+options as shellexecute parameters

links for documentation:
http://kb.mozillazine.org/Command_line_arguments
https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options
there is -new-window option,....

i see the option -setDefaultBrowser to make FF as default browser

-2 Internet explorer
navigate method (+complet syntax) here: https://msdn.microsoft.com/en-us/library/aa752093(v=vs.85).aspx
the flag parameters can be found at:https://msdn.microsoft.com/en-us/library/dd565688(v=vs.85).aspx

object.Navigate(url, Flags, TargetFrameName, PostData, Headers)
can specify url+ optionally flag+window (_blank,_parent,_top,_self or an existing window named ( windowname)).

the same thing is available with the olecontrol embed on a form "shell.explorer.2" (see method navigate,navigate2)
the same things is with internetexplorer.application/navigate method.
if you name the window and set as option ,the ie opens always in this window(if exists).

IE command lines options at:https://msdn.microsoft.com/en-us/library/hh826025(v=vs.85).aspx

-3 from javascript
syntax:
var windowObjectReference = window.open(strUrl, strWindowName, [strWindowFeatures]);

test:(save as test.html and run (fires with default navigator))

<center><a href="" onclick="window.open('http://atoutfox.org', 'myWindow', 'location=yes,height=570,width=520,scrollbars=yes,status=yes');">text open a link in a new window  with defaut navigator</a></center>

myWindow is the window name (can be _blank,_self,_parent,_top or windowname)
if you change the the url its rendered always into the "mywindow" window if exists.


Good bye an happy new year!