Contribute to Firefox or learn Python and C?

How difficult would it be to make decent contributions to firefox in C++, given I'm new to the language? I work as a data analyst programming in R, SAS, and SQL, and have taken a single programming course based on HTDP. My thinking was that learning how to navigate and contribute to a large complex codebase would be a good way to self-teach in an attempt to transition to a software development role. While this would mostly be for professional reasons, I do use firefox and it'd be satisfying to contribute.

I received some advice recently, saying that I'd be better off following my alternative self-taught plan of building a foundation from learning Python, and that I'd be taking on a bit too much trying to contribute to such a large scale project without a solid foundation. I was going to follow what Trevor Blackwell suggests in his FAQ (http://tlb.org/#faq), learning Python and C, and reading through some Python libraries and the interpreter written in C. Python is directly applicable to my job and could perhaps help me land a data engineering role.

Which plan would you suggest? Any idea how long it would take to get to the point where I'd be able to make decent contributions to such a large project? Thanks!