Pros
Once upon a time, Mozilla was ranked as a very good place to work. Once upon a time, it was. I suppose for some careers, like exec assistants, or one of the too many strata of managers there, it still is. I do not know what it would be like to work in Mozilla's Taiwan office. I'm only speaking of Mountain View. As a technology, Rust programming language is starting to look very promising. Comp and benefits are excellent.
Cons
There's a general feeling that people here are least valued where the rubber hits the road. Without engineers and community making stuff, no organization has any use for any of the other roles. But it's clearly the other roles that are more valued. In fact, it's career suicide to work here as an engineer in many areas. The codebase is a mess, there's an awful barrier of entry to contribute to some areas, the way community is treated on projects like Firefox OS (build 'em up, knock 'em down), is despicable. Many wonder, rightfully so, what kind of jobs they can actually get with Mozilla technologies. Good question. They just don't wonder loud enough, in my opinion. The environment has grown recently quite toxic, with backstabbing, etc. I've witnessed somewhat talentless people climbing up the management chain blatantly try to take credit for work others have done. Also, dreadful HR. What purpose do they serve? As it is there is zero path for career growth in many engineering areas, and HR does little if anything to address this problem. The browser share is falling, and Firefox OS market penetration is negligible.