Generally, employees of Mozilla Corp. are paid (and managed) to contribute to specific aspects of the Mozilla project. Sounds confusing, but essentially, imagine there is a new feature for Firefox browser's rendering code that needs implemented -- it's open source so technically anyone can submit a patch, but Mozilla-the-legal-entity may opt to hire and pay you to fix it (and many more after that).

You are then a professional software engineer working for someone who would like to reach a certain goal (such as: A new release of Firefox!) and gives you money to help them achieve it.

Edit: For disclosure, I should have mentioned that I work at Mozilla.