When you move your mouse in a window over text that is a website or email address, or a nickname of a certain type, the mouse pointer changes to a finger indicating that you can click, double-click, or right-click on that text to perform certain functions.
If you double-click on a website address, mIRC will open up your web browser to visit that website.
You can also hold down the shift-key and double-click on email addresses to open your email program. The shift-key is required because of the huge number of addresses on IRC which look like emails but are not.
If you are in a channel window and you move the mouse over text that is a nickname on that channel, you can double-click on it for the usual double-click behaviour, or right-click on it to open the nickname list popup menu. If you single-click on the nickname, the cursor in the listbox is scrolled to that nickname.
You can also double-click on text that is a channel name in any window to join that channel.
If you move the mouse over text that is a nickname in your Notify list, you can also click your right mouse button to pop up the notify popup menu.
Note: This behaviour depends on the Hotlink setting in the Other dialog.