Ah indeed. I'm using WindowsXP/Eclipse3.3.2/Firefox. If I select Firefox
as external Web Browser, clicking Open Web Browser button causes nothing.
If I select Internet Explorer as external browser, clicking Open Web
Browser button opens Internet Explorer. This is my case.
Well, my instant solution is
Goto Window > Preferences > General > Web Browser
Select Firefox and click edit-button. You will see Location such as
"C:\Program Files\Mozilla Firefox\firefox.exe". Edit it to DOS-form path
like as "C:\PROGRA~1\MOZILL~1\FIREFOX.EXE".