Apparently there have been more than a few extensions that were impromptu re-licensed to different source code licenses when forking. Understand that you cannot do this! Many Open Source licenses are incompatible and simply re-licensing an extension under something you personally agree with more is not allowed (and that is a legal not-allowed, in case someone thinks we're "making draconic rules" again).
If you want to release something under your license of choice then you have to write your own extension from scratch. You aren't allowed to pick a license at will when forking because you must stay compatible with the original author's licensing! Otherwise you aren't allowed to use (any of!) the code of the original extension.
While you can, in some cases, re-license extensions, the rule of thumb is that you should avoid it, and only do so if you know exactly what you are doing and have investigated the option to change licensing to what you prefer as legally allowed (i.e. the licenses are compatible for your transition). Simply "upgrading" a license is also not always allowed so you have to be careful when even doing that. In case of doubt, ask your lawyer.
A few examples, off of the top of my head:
- Pure MPL 1.0/1.1 -> any GPL license is not allowed.
- MPL 1.1/GPL 2.0/LGPL Tri-license (a common license for very old extensions) -> MPL 1.1, GPL 2.0 or LGPL license is allowed.
The forked extension will then be subject to the chosen singular license's terms.
- MPL 2.0 -> any GPL license is not allowed.
- GPL 3.0 -> any other license is not allowed. (GPL 3.0 is effectively a licensing dead-end)
- MPL, GPL or Tri-license -> MIT, BSD or Apache is not allowed.
- Pure MPL 1.1 -> MPL 2.0 is allowed with the note you should include "incompatible with secondary licenses" when you upgrade in most cases.
- MPL 1.1/GPL 2.0/LGPL Tri-license -> MPL 2.0 is allowed but you must not include "incompatible with secondary licenses".
- Any Open Source license -> Public Domain is not allowed.