When I enter a single or double quote in mIRC, I get ‘ or “ (Unicode characters) instead of the expected ' or " (ASCII characters.) Since I can type normally in other apps, like this web form, I think it might be a mIRC configuration issue rather than an OS problem.

Does anyone know how to fix this? Thanks.

Based on the advice of maroon and a bit a testing you are most likely getting this:

https://forums.mirc.com/ubbthreads.php/topics/240917/re-double-quotes-arent-showing-up

https://forums.mirc.com/ubbthreads.php/topics/123936/re-smart-quotes

Thank you, that did it! I wonder why only mIRC was affected; I guess it just happens to be the only program I use regularly that obeys that setting.