Tuesday was D-Day for Mozilla with the scheduled launch of the third version of Firefox, its open source Web browser. The company had billed the day as “Download Day” and aimed its sights on setting a Guinness World Record for the most downloads of an application in a 24-hour period. Over the weekend, I dutifully I pledged to download the new browser, and today I anxiously checked the site over and over and over to see if the download was ready. Finally, at 1 p.m. EST, the hour arrived … but still no luck.