Basically, anything. Have a look at http://www.whatcanidoformozilla.org to have a glimpse on what you can do for the project.

Thank you for sharing that. I had NO IDEA that the Mozilla team had such diverse needs, and had always assumed that it was all over my head.

C++, Javascript, Python... or Rust ! :)

The Firefox interface is written in an XML-like language, so you can fix bugs there if you know HTML+CSS.

Or even C, which some parts are written in, although it doesn't look like they're particularly friendly to lots of people changing them (IIRC some of the parsing and lexing code is in C, correct me if I'm wrong).

Most of the C code is in the venerable (Netscape) NSPR and NSS libraries and third-party libraries for media codecs and graphics.