We were playing around with this in channel the other day. Can you confirm that the problem happens only when Firefox is the default browser, and doesn't happen with Chrome? If you're having problems with Firefox doing this, does it make the problem go away to close+reopen Firefox? Someone said the problem went away in the Firefox beta version 57, but I'm not certain whether that was just a result of closing and reopening Firefox.

We were also seeing the problem with
/run www.yahoo.com

so it appears to be a problem with interaction between mirc and the browser, and not specific to just the url window itself or the mouse.