I need to the web address of the largest and most reputable mountain
bike mail/internet discount firm in the US. It Must have a very large
variety of accessories and a large portfolio of brands. It cannot be
bike or equipment manufacturer.

These two are both large, well-known, and I have personal experience
ordering from them.  They provide both mountain and road bike
accessories from many companies and publish print catalogs in addition
to their web sites.

Performance Bicycle: www.performancebike.com

Bike Nashbar: www.Nashbar.com

You cannot go wrong with either of them, but check the prices for a
particular item on both because one is often cheaper than the other. 
Periodically you can find coupons online for them as well.

Sincerely,

Wonko

Great!  Thank you very much.

Nashbar and Performance are actually the same company.  They also
manage Supergo retail and online operations.  For a second or third
company to consider, I would say check out Colorado Cyclist or
JensonUSA.  Best of luck.