Yeah, it's a Linux system. I can't remember if it was spidermonkey that it failed on or not. Though it sounds like maybe they should consider using another JavaScript engine if it's causing problems.
If you can, try again (don't forget to start with `./mach bootstrap` to install dependencies) and open an issue if the build fails. SpiderMonkey is shipped pre-compiled for most architectures, so it's not a big hurdle in terms of build time.