The latest beta is mIRC 7.42 beta1 and can be downloaded here
Note that this distribution does not include a full install of mIRC. It contains only an updated mirc.exe (which you will need to copy over the existing mirc.exe in your mIRC install folder) and a beta.txt file.
This beta includes the following changes:
1.Fixed window focus bug that affected switching windows when using control+tab, the switchbar, and other methods.
2.Added $sysdir(item) to return system folders for the current user where item can be profile, desktop, documents, music, pictures, and movies.