Its under the Mozilla Public Licence, see [link] The most pertinent requirement as stated in the FAQ:-
"I want to distribute a modified version of Firefox (or other MPL-covered code). What do I have to do?
You must add a conspicuous notice stating where to find the exact source to the binary you are distributing. If you wish, you may point at mozilla.org for the base code and then ship diffs between our version and yours."