It’s after 5 pm EDT, and Firefox Download Day is well under way. The site listed over 1.3 million downloads so far, only a few short hours after opening the servers to the masses.

Like many tech projects, Firefox Download Day gave Mozilla’s engineers some last-minute worries. Eager downloaders slowed Mozilla Foundation servers to the point of inaccessibility for a brief period of time as the expected start of the record-download tracking approached.

“The outpouring of interest and enthusiasm around Firefox 3 has been overwhelming (literally!),” Melissa Shapiro said earlier at the Mozilla Blog.

Our download and installation of Firefox 3, replacing the most recent full version of Firefox 2, proceeded briskly. Some add-ons for the browser may need an update from their developers for compatibility with Firefox 3, as we discovered; we expect maintainers of the most actively developed add-ons to respond quickly.

One of those, the browser add-on StumbleUpon, should have a new version available in the next few days to function with Firefox 3.

“The Firefox 3 Smart Location Bar will allow StumbleUpon users to quickly access their past favorites by typing text into the location bar where the auto-complete function will begin to display matching Stumbles,” StumbleUpon said in a statement.