Sorry to disappoint you turboke but the /run problem you describe is NOT a mIRC bug, it is caused by a known bug in Firefox 0.9.x and Thunderbird 0.6 as they do not correctly register themselves as default protocol handlers.
If you use the "Run..." dialog in the start menu and enter a URL (http, ftp, https and gopher protocols are all affected) then you will get an error dialog which says
"Cannot find the file http://xxx.xxx.tld (or one of its components). Make sure the path and filename are correct and that all required libraries are available".
Details of the cause can be found
here and a patch,
here. Once you apply the registry patch the /run command in mIRC will function correctly as will opening URLs in the explorer shell.
EDIT: Forgot to add my software setup as later versions of Firefox may be fixed. Win2k, mIRC 6.14, Firefox 0.9.1
