STR: 1. be on osx or somewhere else with level-3 sandboxing 2. ./mach build with: mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/../builds/opt/ ac_add_options --enable-warnings-as-errors or even with: mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/../builds/frontend/ ac_add_options --disable-compile-environment ac_add_options --enable-artifact-builds 3a. ./mach mochitest browser/base/content/test/urlbar/browser_urlbar_stop_pending.js ER: the test passes AR: you get stuck on the first ContentTask.spawn() which seems to, uh, do absolutely nothing. Or something. 3b. ./mach mochitest browser/components/extensions/test/browser/browser_ext_browserAction_popup.js --tag remote-webextensions ER: the test passes AR: you get stuck somehow (don't know this test as well, so no idea what's happening). passing --setpref 'security.sandbox.content.level=2' makes these tests pass. To say this is annoying is an understatement, especially when you're working on a patch that's causing timeouts on try, so seeing timeouts with your patch is expected, but then it turns out the same timeouts happen without your patch, and then you're just very confused and waste a lot of time blaming other people/things/pixies, or, in Great British tradition, the weather.

I can only assume that because tests on infra run against packaged builds and tests, they don't run into the same issues.

This sounds like a dupe of Bug 1381517 "Level 3 sandboxing for OSX builds with obj-dir outside of source-dir breaks PeerConnection mochitests" which we hope to fix very soon with either Bug 1380690 or Bug 1380416. In the meantime, please try this workaround setting the REPO and OBJ dir environment variables manually. $ MOZ_DEVELOPER_REPO_DIR=/path/to/central MOZ_DEVELOPER_OBJ_DIR=/path/to/obj/dir ./mach mochitest <test>

With the fix for bug 1380690, the workarounds should be no longer necessary. Closing as a dupe.