As with most software, new versions of mIRC are released every so often that add new features, fix bugs, and address security issues.
We generally recommend using the latest version of mIRC because it can include changes that address important issues.
Until recently, the mIRC website provided a download link for an older version of mIRC, mIRC v6.35, and recommended it as the only safe version for users wanting to use an older version of mIRC. All versions older than mIRC v6.35 are open to exploits. An exploit allows someone to gain access to your computer without you realizing it.
However, recent security fixes mean that mIRC v6.35 is now also not safe. Due to the number of bug and, especially, security fixes, we can no longer recommend using any older versions of mIRC.
The only version we can recommend using is the latest version available from the official mIRC download page