You have three typos in your code:
Should be be written as follows:
<!DOCTYPE html PUBLIC "-//W3C/[B]<font color='Magenta'>/</font>[/B]DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD[B]<font color='Magenta'>/</font>[/B]xhtml<font color='Magenta'>[B]1[/B]</font>-strict.dtd">
I'll give you a tip for the DOCTYPE, i.e. this code sample in this post copy and paste a correct Doctype (those three lines) rather than typing it all by hand - not even I bother typing out that part in full... I've highlighted the 3 corrections too.
However, that won't effect it from rendering in Fx. thus most probable the reason you are seeing markup and not the actual webpage is probably because your file extension is still *.txt and not *.html You might have accidentally have a file called: basic.html.txt
One easy way to findout, is to look at the Firefox address bar URL and see if the file you load ends in *.html your file should be called basic.html and nothing else.
Double-check it has the *.html extension as sometimes people accidentally leave the extension *.txt in place on the end when is should be be *.html