Our bug tracker contains a "good first bug" label, which you can use to find good first bugs to work on.
Gecko:DeCOMtamination: Convert weird, ugly and inefficient method signatures to more natural C++ method signatures. It really improves code readability, reduces code size, and increases performance. It's a bit tedious but it's extremely valuable and will help you learn the way around the codebase. We've done a lot, but there's a lot more to do. Anyone with reasonable C++ experience can do this.