Pros
If you're starting your dev career, this is the place to cut your teeth. Everyone here is deeply passionate about their work, and you'll learn a lot about how to design great software thanks to the openness of the code review and design process. Great place for independent learners, and people with great dev potential/skill and organizational skills. Must be an autonomous worker. You'll drag everyone down and your own self-esteem if you can't learn things on your own. Pay and benefits are great.
Cons
Mozilla is bleeding talent for many reasons, all of which would affect senior engs/management. Scapegoating, not identifying real leadership vs those who are deceitful, and not accepting blame. As a result, I've seen innocent lead engineers/maangers get blamed for things they didn't do, and get pushed out of the company for it. It's been disgusting and demoralizing. There are favourites and then there are the black sheep and these politics are breaking Mozilla. Some people who shouldn't be leaders or management get into these roles, and it damages the company because they can't handle the organization skills involved to deliver goals they commit to, and/or they lack the personal skills to deal with inter-team morale issues. Transparency doesn't exist here anymore.