Pros
In my time at Mozilla, I've only become more impressed with how the company works. People/engineers/devs (especially those more senior) mostly set a tone of openness and a willingness to help others. Mozilla as an employer has good benefits and in my opinion strives very very hard to do right by their employees. Whether that is supporting you with your career goals and aspirations or providing budgets for conferences, classes, or tools to you help you. From my perspective, part of Mozilla's culture is to be informal leading to a more startup get-it-done atmosphere. Web browsers are big complicated projects and there are many challenges/opportunities where good ideas and hard work are needed.
A lot of Mozilla's people work remotely and Mozilla supports remote employees very well by giving you all the tools you need. As a software engineer working on a team of about 10 people that are also mostly remote, I don't feel that being remote is an impediment to my career or getting work done. With company news, presentations, or announcements, there is always an option to listen and participate remotely via video chat, etc. All meetings seem to be conducted in a remote-friendly manner.
My manager wants to make sure we feel good about what we're working on and often asks for our opinion on changes, strategy, etc. Every month the CEO presents to the company about how we're doing and there is always opportunity to provide feedback with Q&A (including anonymously). There are weekly presentations about what's going on with Firefox. Guest speakers from the tech/privacy/governance visit and present on various topics. Company-run offsite all hands are extremely well done.
Mozilla's mission is a good one. I couldn't imagine a better place to work
Cons
There are many high performers here and that comes with certain pressures you may feel or place on yourself.
Everything Mozilla does will be misrepresented or hated by someone.
Web browsers are big complicated projects and building knowledge/ramping up takes time and effort.