Every year in October the developers from around the world have the Hacktoberfest (this not mean that they don’t attendee also an octoberfest!).
The Hacktoberfest is an initiative between Digital Ocean and GitHub where developers in October have to do at least 4 pull request to receive as gift a limited edition t-shirt and few Github stickers.
How to join?
Very easy, go to https://hacktoberfest.digitalocean.com/ and subscribe with GitHub.
Automatically they will check your profile and in case you done the 4 pull request or more you will be contacted to leave the shipping information for the swag.
This year will more interesting because there is a list of tickets with a label hacktoberfest, to find suggestions or somewhere to start.
I already added in my personal project new ticket or in old that label and I already received in few hours few reply.
So if you have few ticket or need features join the opportunity where others can work for you and at the same time you can work on other projects.
Contribute to Mozilla
To contribute is easy usually Mozilla can use Bugzilla for few projects for the tickets but the code is on GitHub, or there are projects already on GitHub like the others.
Contribute to WordPress
In WordPress the situation is different, many plugins exist on GitHub but to contribute to WordPress itself is not possible.
They have a mirror https://github.com/wordpress/wordpress that accept pull request but they are rejected but with this tool is possible to convert a pull request as patch and add it on a ticket https://rmccue.io/patch/.
To find tickets for wordpress look on https://make.wordpress.org/core/reports/.
See you on GitHub, maybe also on my projects or others!