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As the title says.

So there's this open source project I'm forking. The original was released under GPLv3.

Now I want to introduce additional features based on a different open source project, but this second project is released under MPL-2.0

Is that legally allowed?

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According to the MPL 2.0 FAQ in Q14, Yes, with three requirements

  1. The MPL software isn't incompatible with GPL, either because the original author specifically mark it as such, or it was originally released on MPL 1.1 without dual/tri-license with GPL
  2. It's actually combined with GPL software, instead of merely switching the license.
  3. The MPL-licensed part of the source code is available to your direct recipient under both MPL and GPL. Those from their downstream can elect to use MPL-only or GPL-only.

Since in your scenario point 2 is clearly covered, all you need is to ensure the second project never specifically excluded from licensed under GPL by the original author and you're willing to provide the MPL part under both MPL and GPL.

The GNU's GPL compatibility note confirms the compatibility

Section 3.3 provides indirect compatibility between this license and the GNU GPL version 2.0, the GNU LGPL version 2.1, the GNU AGPL version 3, and all later versions of those licenses.

and gives a specific example for point 1

Parties who release original work under MPL 2.0 may choose to opt out of this compatibility by including a sentence in the license notices that says that the work is “Incompatible With Secondary Licenses.” Any software that includes this notice is not compatible with the GPL or AGPL.

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