I just saw this on the Mozilla site:
Obviously software can be distributed under GPL, LGPL, etc. at the same time.Core Mozilla project source code is licensed under a disjunctive tri-license giving you the choice of one of the three following sets of free software/open source licensing terms:
* Mozilla Public License, version 1.1 or later
* GNU General Public License, version 2.0 or later
* GNU Lesser General Public License, version 2.1 or later
This allows the use of our code in as wide a variety of software projects as possible, while still maintaining copyleft on code we wrote.