Thanks for looking into it.
To set up emeditor as default source viewer:
In firefox 3’s address bar, type about:config
search for view_source.editor.external
set the value to true
Then in “view_source.editor.path” , input the path to emeditor.
And to reproduce this issue, viewing source of many page with non-alphanumeric titles will fail to open directly
For example, http://b.hatena.ne.jp/
I reproduced your issue, but it is not related to EmEditor. Notepad.exe, for example, would not open correctly either.