First, Mozilla has and does assist in the creation of FOSS game engines and tools (Gladius, CubicVR, help to various web ports like JSMESS and various open source game engines, etc.).
Second, even the LGPL shows that absolute avoidance is not always the best thing to do regarding closed-source code. Reality is more complex. Sometimes interoperability with closed-source code is good for everyone including free software.
In particular, a big benefit for free software here is that Unity games can now run on the web, an open platform. Before they required a proprietary plugin that did not work on Linux or BSD, but as JS+WebGL they can run on those platforms. (Yes, the games are proprietary, but visiting websites using proprietary code is basically unavoidable, both on the client and server-side. At least we can do it using FOSS tools now.) So this helps support free software OSes.
A final point, while the Unity engine is a proprietary codebase, they also have lots of code that is FOSS, see their github page. They are also working with even more open source code for the WebGL port, as it uses emscripten.