That’s the Mozilla Public Licence. If I recall correctly, it is considered incompatible with the GNU GPL (although it probably shouldn’t be) without an explicit clause permitting combination of MPLed software with GNU GPLed software.

The MPL is a superb licence that is under-appreciated. It usually acts similar to the MIT licence in that it never taints your software that uses MPLed software. It has, at best, a “modular” taint. If you depend on an MPLed library, you can combine that library without revealing your source at all. If you modify that library, you only have to release your modified source. You can even modify it so that it goes from proprietary -> MPLed library -> proprietary, but you have to document the nature of what you’re doing in that call out from the MPL and you may need to release that (I haven’t dealt with the MPL in years, but I think that describes that particular case).

GNU activists were loud enough for years (despite having had the exact same debate at the beginning of the licensing project) that ultimately Mozilla caved and dual-licensed its projects just to shut them up. (It didn’t help with some folks in the Debian community because of the constraints on the trademarks, but some people will never be satisfied unless you’re receiving your web pages by email in emacs.)

Thank you. I asked because the expression "MPL with GPL exception" wasn't clear to me.

You may want to know that in the past few years, MPL 2.0 was released, which embeds a so-called "GPL compatibility" by default. In fact, it's a relicensing allowed by MPL 2.0 licensor.

MPL 2.0 still keeps an option for the licensor's software to never be relicensed, but they have to specifically use an "incompatibility" clause.