Awesome. Works like I want it to
DU, everyone else, thanks. I hope some of you guys use it.
Here was apparently the problem:
I opened up a pre-existing search engine file, and noticed that it had a particular font (or lack thereof) in the text editor. When I originally saved the file, I selected .xml from a pull-down menu and it saved a file that when opened, had a different font. Apparently the default behavior of TextEdit is rich text format (RTF) and it seemed to save it in that format as well, but I'm not really familiar with .xml so I didn't exactly know. I went into preferences and selected plain text as the default new document format. I then saved the DU's code verbatim into the document, saved it as .xml by typing .xml at the end of the filename, and hit save.
Restart firefox and ta-da! McMaster Carr, just a right click away!
Go OSS (Open Source Software)
Edit:
PS, this also killed two birds with one stone. I did not like the fact that TextEdit's default behavior was RTF. I could save as a .txt file from a pull down menu, but now I'm much happier with TextEdit as well.