Every new and existing file in Bugzilla must be licensed under the Mozilla Public License, v2.0. Generally, you can look at the existing examples at the top of files similar to the one you're editing. Files should contain the following license, near the top:
# This Source Code Form is subject to the terms of the Mozilla Public # License, v. 2.0. If a copy of the MPL was not distributed with this # file, You can obtain one at http://mozilla.org/MPL/2.0/. # # This Source Code Form is "Incompatible With Secondary Licenses", as # defined by the Mozilla Public License, v. 2.0.
For more details, see the Mozilla licensing page, except remember that Bugzilla is incompatible with secondary licenses.