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There's no need to overthink how you're going to contribute works on all levels. Instead, consider some of the projects you already use and how you may improve or change them.
According to research, documentation, typo repairs, and even translations account for about 30% of all casual contributions.
If you are a first-time contributor – which everybody is at some point – here are some links that will get you started in the open source world:
Mozilla Specific: What Can I Do For Mozilla
As I say, "You need not to be an expert to contribute to Open Source". You can get started with these projects and make your first Open Source contribution.