Pontoon is an interesting project from Mozilla , I have done a blog for how to install and what you can use it for .
Since its an open source project , the source code is available via GitHub :
https://github.com/mozilla/pontoon
As a contributor you can help in make this product better by one of the following :
1- contribute as a localizer/translator .
2- contribute in fixing bugs .
To contrubite as a localizer , you can follow this link :
https://pontoon.mozilla.org
Create an account and you will be able to suggest strings to projects .
you can see the completion level and how important “priority level” of the project by visiting this :
https://pontoon.mozilla.org/projects/

The other way you can contribute to Pontoon by fixing bugs , and the list of bugs are available in bugzilla and you check it out from here :
https://bugzilla.mozilla.org/buglist.cgi?component=Pontoon&product=Webtools&bug_status=__open__
you will have to contact the admins of the project to assign you to the bug and they will help you out with it , the team very helpful and cooperating (Thank you Matjaz!) .
The screenshot below to shows the bug i have worked on :

After that ensure that you clone the project from GitHub , and ensure you follow the contribution guideline you can find all the information here :
https://mozilla-pontoon.readthedocs.io/en/latest/dev/contributing.html
few things to keep in mind :
1- Create a branch for the bug that you are working on in your local repo .
2- run the testing locally since once you do pull request the code will be checked via
https://github.com/travis-ci/travis-ci
3- Pontoon is written in Python , so you should be familiar with Python and Django framework.
I do following these steps in order to test my code :
1- ensure you are running the virtual environment which is :
virtualenv venv
source ./venv/bin/activate
this is mentioned once you run pontoon in your local server and you can read more about this in the installation page for Pontoon .
2- run pylama command which will check for the code conventions .
pylama pontoon
once you do pull request and your branch for the bug passes the testing , and the code satisfy admin , your code will be merged . You may receive comments to change certain things in the code by the admin before they merge your code .

Its very interesting experience , and actually you will be enjoying working on it .