Local builds on computers with some antivirus software will fail to build after quarantining security-related tests, with an ERROR PROCESSING MOZBUILD FILE. For example: Kaspersky has quarantined media\libstagefright\gtest\test_case_1216748.mp4 on my machine. I'm leaving this bug here to get some sunlight for now, but I'm inclined to make this a documentation problem and say "if you have this problem: whitelist the repo, hg update -C and try again." If nobody has any better ideas, I'll take the bug and update the relevant docs.
You're lucky you got to a build: we've had A/V corrupt VCS clones because it detects a virus in a test case. A/V (including Windows Defender) can make many developer operations much slower (including building and running tests). We really need to better surface the need to disable A/V on your source and object directory if you care about optimal performance. See also bug 1272851.
Updated the docs here to include a paragraph about AV whitelisting and performance impacts. https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Windows_Prerequisites