mIRC can display windows in two ways:
1) If the windows are displayed inside mIRC, which is the default, they will be MDI windows and will have thick borders. The "Cascade" feature is used to arrange MDI windows inside mIRC.
2) If the windows are displayed outside of mIRC, they will be desktop windows and will have thin borders.
If you were previously seeing windows with thin borders, this means that you set up mIRC to display your windows as desktop windows.
You can set them to display as desktop windows again either:
1) By opening the View menu in the menubar at the top of mIRC and selecting the Mode/Compact option.
2) By opening the Options dialog and, in Display/Options/Windows, setting windows to open as desktop windows.