First of all, Mozilla is a wider open source community in the world. Open source is that where one can explore many new things like view source code of any open source project like Firefox browser, Firefox os and many more and get recognization in the community.
Great Opportunity to Learn Something New: You will be facing the problems while writing a code for open source which you are likely not encountered before, which present better learning opportunities. You can try out new tools and techniques on an open source project. For example, if you've never done any unit testing, and cannot get permission from your boss to do so in your regular job, writing some code for an open source project would be an excellent place to try it out. Collaborate with Really Smart People when you are working with open source, you will really get to know many people around the world who have similar interests. Moreover, open source organization like Mozilla has many contributors who can help you in solving any problem within no time.
Interesting and Challenging: Contributing to open source can get us with many challenges which we haven't faced before and writing a software which is useful to masses is really enthusiastic.
It Looks Good on a Resume: More and more employers of developers are looking at community contributions. They like to see that you're blogging and contributing to mailing lists, and they really like to see you listed as a committer on an open source project. It can also help with the work experience section of your resume.
What can I get by contributing? Explore many new things, get recognization in the community, get splendid gift packages, get a free ticket to any Mozilla related events, get an intern. Moreover as said above, get a job easily as more companies look for if you are an open source contributor before.
How to contribute to Mozilla ? There are many projects for which you can contribute to Mozilla some of which are as follows:
Coding: If you are interested in coding related stuff, You can contribute to the Mozilla code base, Firefox os apps, addons, thunderbird, webmaker .....and many more.
Add ons: Mozilla products have a powerful extension system that allows you to make small tweaks or major additions. Learn more about how to build an add-on.
Web development: Can help in buliding websites for Mozilla
Localization: Get involved with Mozilla by making Firefox, Thunderbird and other projects available in your language.
Helping users: Help other users in solving a problem.
Documentation: The Mozilla Developer Network is the hub for information about Mozilla technologies. Help make documentation better by writing new content, correcting existing material or translating content into new languages.Education: Help build a generation of webmakers by teaching others how the web works. Explore what the Mozilla Webmaker project is all about or get started teaching others by setting up an event in your community today.