I am developing this days a WebExtension and my work is based of some external libraries, all with different licenses.
One is “Apache 2.0”, other is “GPL v3” and the last is MIT licensed.
What is the most permissive license that may be used for my extension?
I’m afraid your extension must be under “GPL v3”.
As an alternative, if this is possible in your case, you can put the GPL code into a native application (under GPL) called by from the add-on and in that case, your extension can run under any license you choose.
After some researching, I found that the GPLv3 library has an older version that is MIT licensed and this may be enough for my needs.
I found this older version in a different place than the official site (sometime ago the library has switched from MIT to a dual comercial/gplv3 licence).
It’s legal / ethical to use the older version?