Upstream watchman has gotten much better Windows support now (even considering it Beta-level support), as has fsmonitor. Let's try updating watchman and see if we can enable fsmonitor by default now without breaking the world.
Pushed by firstname.lastname@example.org: https://hg.mozilla.org/mozilla-build/rev/846e3102cb6b Update watchman to upstream revision 95ac0c35. https://hg.mozilla.org/mozilla-build/rev/b7a025509390 Enable fsmonitor by default. r=gps
fsmonitor.mode defaults to "on." So we could remove that entire section if we wanted to.