1. Text editor doesn't work in Firefox because Firefox doesn't use Microsoft's activeX plugin which can potentially allow someone else to take over your browser and run programs and scripts on your PC which could be nasty/malicious.
Windows Updates is a benevolent example of this : it installs improvements on your PC using activeX.
2. You can select text in Firefox because that browser ignores Microsoft's proprietary code. In HTML you can either stick to the code that everyone agrees on (the w3c sets the standards) or, if you're Microsoft you can make it up as you go and invent new code that only works in your own browser, e.g. this "unselectable" (that you might see when someone's posts includes lots of html code :
http://www.theanswerbank.co.uk/Sport/Question170730.html
).