The "lemmings" want something that simply works, they are not interested in the details. It's like their car, they are not interested in the type of oil used or the brand of the spark plugs.
They want to read their webmail, get some info from the web, buy from the nearby store online, plan their holiday, sell/buy a few things on eBay, and manage their bank account online.
Now, some of these sites are IE-only. This means that when I put Firefox on their computer they are going to be angry at me to have installed a defective product on their machine, or they just silently shift back to IE. Plus, they have memorized the keyboard/mouse patterns for IE, and it's not 100% compatible with Firefox. I tell them to use Firefox in general and IE for those IE-only sites, but after a while they go back to IE because it's inconvenient to switch back and forth.
"Could you come and have a look? I have this program that pops up and wants me to go to this porn site. Also, the computer is very slow and freezes all the time again, I don't understand why."