a) it's simply as clumsy as could be.
b) it's GUI is not in synch with what seems natural and intuitive to me, while the FF GUI does.
c) it's simple, fast & easy to adjust the FF "Options", which are laid out intuitively (to me), while it's a trial & error PITA to adjust IE's "Tools" and "Internet Options" to achieve what I want.
d) it takes time I don't have to pause and figure settings out for IE. Figuring out how to make the settings I want in FF takes only a few seconds, i9s nearly efortless.
e) it labels things using a vocabulary that's pretty much unique to Microsoft's proprietary world, which again is a PITA. FF labels things using a (to me) more natural vernacular, and which seems much more "universally" proliferated in the universe of computer software applications. For example, everybody says "bookmarks." Nobody says "Favorites" except Microsoft, so a user has to learn the translation that "Favorites" means "bookmarks" which is what everybody says.
f) there are only 2 fans of IE on God's earth: forum Member mechBgon (j/k! :laugh & Steven Anthony Ballmer. The rest of us only suffer the use of it when dealing with banks and MS updates.