After more than a year of missteps by Apple's tablet rivals, at least one viable competitor, Amazon's Kindle Fire, appears to have surfaced for the popular iPad. Amazon.com Inc. on Monday trumpeted the success of its recently launched Kindle Fire tablet, part of a family of Kindle products that include low-priced e-readers. The company said it sold more than four times as many Kindle products on Black Friday last week as the same shopping day in 2010.