I got a quick response from Mozilla's HR person after applying. Had an hour-long phone interview with her that went very well. Had a second video conference interview the following week with the engineering manager. He seemed more concerned about my age than my ability to contribute to their development efforts. And then the following week, I had a third video conference interview with their technical lead. He had me optimize a small program. I thought I did OK. The interviewer acted like he didn't want to be there and was just going through the motions. Two days later I got a rejection email from the initial HR contact with no explanation as to why I was turned down.