It’s a bit more complicated than that because MPL is itself a weak copyleft license that requires that the MPL licensed source code is always made available to recipients of a binary or derived work. The difference from GPL is that it does not require that all additional parts of the derived work are also licensed under MPL, (ie. not viral copyleft) meaning that the MPL licensed work can be linked with proprietary code without requiring that the proprietary code make its source available, but unlike BSD or MIT licenses it does not allow the MPL licensed code to be made proprietary.
The complication comes when linking MPL code with GPL code, even though MPL is GPL-compatible, since this requires that the entire derived work must now be made available under the GPL, while the original MPL licensed parts become dual-licensed under both MPL and GPL.
If Waterfox developers allowed this then it would prohibit the use of the whole derived work in proprietary projects (as they would now need to be GPL), so it would be removing rights that they have already given to downstream users of their code. Proprietary projects would therefore have to remove the GPL licensed additions (in this case it would be the UblockOrigin code) and link just the MPL licensed parts, which would mean using only part of the whole browser.
Personally I agree with you: I prefer GPL licensed projects. But MPL is not a bad license and I can understand and respect that some developers would make that choice (especially since the project is already licensed under MPL as it’s a fork of Firefox).
It’s a bit more complicated than that because MPL is itself a weak copyleft license that requires that the MPL licensed source code is always made available to recipients of a binary or derived work. The difference from GPL is that it does not require that all additional parts of the derived work are also licensed under MPL, (ie. not viral copyleft) meaning that the MPL licensed work can be linked with proprietary code without requiring that the proprietary code make its source available, but unlike BSD or MIT licenses it does not allow the MPL licensed code to be made proprietary.
The complication comes when linking MPL code with GPL code, even though MPL is GPL-compatible, since this requires that the entire derived work must now be made available under the GPL, while the original MPL licensed parts become dual-licensed under both MPL and GPL.
If Waterfox developers allowed this then it would prohibit the use of the whole derived work in proprietary projects (as they would now need to be GPL), so it would be removing rights that they have already given to downstream users of their code. Proprietary projects would therefore have to remove the GPL licensed additions (in this case it would be the UblockOrigin code) and link just the MPL licensed parts, which would mean using only part of the whole browser.
Personally I agree with you: I prefer GPL licensed projects. But MPL is not a bad license and I can understand and respect that some developers would make that choice (especially since the project is already licensed under MPL as it’s a fork of Firefox).