v6.10 on Win98SE.
View > Menubar... (hides Menubar)
Right-Click Titlebar > Help > About... (shows About dialog)
Cannot close window via Continue button or Alt+F4.
(note: Unconventional means exist to WM_ENABLE the main mIRC window so you can close it gracefully. Downlaod CopyText
if this interests you, a handy little app.)