It's not really possible for channel windows to be manually opened (and empty) before mIRC is connected to a server. IE, upon starting the program, like a reopened browser tab while offline.
mIRC can be configured to automatically join channels upon connect, either via Favorites, or via script.
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If you want more persistence in your IRC experience between reboots, and even across multiple computers, then you can install a bounce(BNC)-proxy software on a remote linux computer, which stays connected to your favorite servers and channels, and will act as your own private IRC server to connect to. ZNC is the most popular of these BNCs, and you will require ownership or lease rent on somebody else's linux server.
