The entire process took 3 months, most of which was following up and not hearing anything back.
I applied in January, and got an interview in February with HR which very quickly resulted in an interview with the hiring manager.
During the interview with the hiring manager, they pinpointed a position on their team they thought I would be a better match for, and I agreed. The hiring manager was incredibly polite, and seemed really focused on what parts of the job I personally enjoy, which I found a wonderful change of pace from the more common "what can you do for us" questions I'm accustomed to in interviews. I heard back quickly that I had advanced to the next round for that role, but that it couldn't be scheduled right away. I was okay with that, figuring they want to keep candidates fairly even throughout the process. Unfortunately that was only the start of issues getting ahold of the team.
I was eventually contacted for that third round after a few weeks of follow-ups, which consisted of a total of four interviews each 1:1 with the team, in early March. I submitted my availability, but didn't hear back. I followed up the next week, and had to follow up again after that, finally getting an interview scheduled 3 weeks after I had submitted availability. This was rather frustrating for me - I had been informed it was a 2.5 hour block of time on my calendar, and trying to leave that much time open when I am over 8 hours of meetings per day in my current job was next to impossible.
Eventually the interviews did get scheduled, but were separated out rather than all being at the same time, which I strongly preferred. The first three interviews went smoothly, with each person asking a series of questions I felt were very in line with the role, and making me feel very welcome. I loved meeting with those individuals and am truly sorry things didn't work out, as I think they would have been great coworkers. The final interview of that stage got rescheduled for a full week out, and then rescheduled two more times. Finally the person provided a single day and time, during which I was not free, but I made it work anyway fearing I might never get another chance. In the end, it took almost a month to get an interview with that last person, who seemed to have no idea who I was or what position I was applying for. They even told me there were no positions on the team and seemed confused about why I was interviewing.
From there, I begin following up with HR for my next steps. During this time, I saw the job posting come down, then come back up with a second position of the same type on the same team. I followed up with HR every week for 3 weeks until finally being written back and told the position was cancelled.
It saddens me greatly that Mozilla did not have the courtesy to respond to most of my follow-up emails during this process, and to see that they are so disorganized they would make an extra position and then cancel it entirely. I had been really looking forward to the possibility of working with this company and now, I'm so hurt by this experience I can't even have Firefox as my default browser anymore.
I would like to state that all the people I interviewed with in the company were wonderful, and the individual interviews went smoothly, but the communication with HR is some of the most negative I have ever experienced with a company, which is why I have to rate this as an overall negative experience. If I could have spoken directly with the hiring manager, I think things would have been better.