trying to run a .html file from windows explorer gets me a error message that reads "there was a problem sending the command to the program."
This is the crux of your problem. It appears that either your default browser is no longer set as your default browser, or it's mis-behaving. Try going to INTERNET EXPLORER and go to tools/internet-options and the programs tab, and click on "make default". This -should- make windows-explorer launch an .html into IE, and should allow mirc to push a link into IE. (it's possible that you might need to restart mirc before it sees a change in default browser, i'm not sure).
If that fixed the problem, you can go back to firefox and re-select that browser as your official default browser. If the problem was solved with IE but appears again after switching back to firefox, your firefox setup might need fixing or reinstalling.
When you put "there was a problem sending the command to the program." into google, you find lots of hits about this problem.
In summary, your problem is in windows itself - mirc is merely how you are stumbling across this problem.