I don't know if it's practical to compare mIRC with applications such as Opera, Firefox or Explorer, which are used for a different purpose, present information in a different way, and have a different level of interactivity.
mIRC is designed to be frugal with resources - however there's only so much it can do when it comes to windows resources where it has to open a new MDI window for every window type that you need, along with any other required resources such as edit box, list box, and scroll bar windows, as well as the supporting infrastructure for that window type. Most other applications, such as browers, do not use MDI windows and do not display more than a single window of information at a time. There are of course no plans to remove MDI support from mIRC. Other applications may also avoid use of resources by using their own custom-drawn resources used instead of standard windows resources. There are no plans to do this in mIRC either. In fact I plan to move away from the current custom-drawn resources in mIRC to standard windows resources in future versions, which may require more windows resources.
Overall, I think it's impressive that mIRC can open that many server, channel, query and custom windows at the same time - the amount of information that's being stored and processed on a per connection/window/event/user/script basis is significant - it's an extreme situation and is probably an indication of how stable mIRC is.