We have been working to get runByDir enabled, and this has turned up on several pushes with runByDir enabled, intermittently. Here's one such push : https://treeherder.mozilla.org/#/jobs?repo=try&revision=07724104ec8f&filter-resultStatus=testfailed&filter-resultStatus=busted&filter-resultStatus=exception&filter-classifiedState=unclassified 10:13:11 INFO - dir: toolkit/components/url-classifier/tests/mochitest 10:13:11 INFO - Setting pipeline to PAUSED ... 10:13:11 INFO - Pipeline is PREROLLING ... 10:13:11 INFO - Pipeline is PREROLLED ... 10:13:11 INFO - Setting pipeline to PLAYING ... 10:13:11 INFO - New clock: GstSystemClock 10:13:11 INFO - Got EOS from element "pipeline0". 10:13:11 INFO - Execution ended after 32697942 ns. 10:13:11 INFO - Setting pipeline to PAUSED ... 10:13:11 INFO - Setting pipeline to READY ... 10:13:11 INFO - Setting pipeline to NULL ... 10:13:11 INFO - Freeing pipeline ... 10:13:11 INFO - INFO | runtests.py | ASan using symbolizer at /builds/slave/test/build/application/firefox/llvm-symbolizer 10:13:11 INFO - INFO | runtests.py | ASan running in low-memory configuration 10:13:11 INFO - pk12util: PKCS12 IMPORT SUCCESSFUL 10:13:12 INFO - INFO | runtests.py | ASan using symbolizer at /builds/slave/test/build/application/firefox/llvm-symbolizer 10:13:12 INFO - LSan enabled. 10:13:12 INFO - LSan using suppression file /builds/slave/test/build/tests/mochitest/lsan_suppressions.txt 10:13:12 INFO - INFO | runtests.py | ASan running in low-memory configuration 10:13:12 INFO - INFO | runtests.py | ASan using symbolizer at /builds/slave/test/build/application/firefox/llvm-symbolizer 10:13:12 INFO - INFO | runtests.py | ASan running in low-memory configuration 10:13:12 INFO - MochitestServer : launching [u'/builds/slave/test/build/tests/bin/xpcshell', '-g', '/builds/slave/test/build/application/firefox', '-v', '170', '-f', '/builds/slave/test/build/tests/bin/components/httpd.js', '-e', "const _PROFILE_PATH = '/tmp/tmpu5s23k.mozrunner'; const _SERVER_PORT = '8888'; const _SERVER_ADDR = '127.0.0.1'; const _TEST_PREFIX = undefined; const _DISPLAY_RESULTS = false;", '-f', '/builds/slave/test/build/tests/mochitest/server.js'] 10:13:12 INFO - runtests.py | Server pid: 12092 10:13:12 INFO - runtests.py | Websocket server pid: 12095 10:13:12 INFO - INFO | runtests.py | ASan using symbolizer at /builds/slave/test/build/application/firefox/llvm-symbolizer 10:13:12 INFO - INFO | runtests.py | ASan running in low-memory configuration 10:13:12 INFO - runtests.py | SSL tunnel pid: 12100 10:13:13 INFO - runtests.py | Running tests: start. 10:13:13 INFO - runtests.py | Application pid: 12122 10:13:13 INFO - TEST-INFO | started process Main app process 10:13:13 INFO - Xlib: extension "RANDR" missing on display ":0". 10:13:25 INFO - SafeBrowsing: 10:13:25 GMT-0700 (PDT): getLists: urlclassifier.phishTable 10:13:25 INFO - SafeBrowsing: 10:13:25 GMT-0700 (PDT): getLists: urlclassifier.malwareTable 10:13:25 INFO - SafeBrowsing: 10:13:25 GMT-0700 (PDT): getLists: urlclassifier.downloadBlockTable 10:13:25 INFO - SafeBrowsing: 10:13:25 GMT-0700 (PDT): getLists: urlclassifier.downloadAllowTable 10:13:25 INFO - SafeBrowsing: 10:13:25 GMT-0700 (PDT): getLists: urlclassifier.trackingTable 10:13:25 INFO - SafeBrowsing: 10:13:25 GMT-0700 (PDT): getLists: urlclassifier.trackingWhitelistTable 10:13:26 INFO - 3258 INFO TEST-START | toolkit/components/url-classifier/tests/mochitest/test_allowlisted_annotations.html 10:13:28 INFO - MEMORY STAT vsizeMaxContiguous not supported in this build configuration. 10:13:28 INFO - MEMORY STAT heapAllocated not supported in this build configuration. 10:13:28 INFO - MEMORY STAT | vsize 20972770MB | residentFast 460MB 10:13:28 INFO - 3259 INFO TEST-OK | toolkit/components/url-classifier/tests/mochitest/test_allowlisted_annotations.html | took 2309ms 10:13:28 INFO - 3260 INFO TEST-START | toolkit/components/url-classifier/tests/mochitest/test_classified_annotations.html 10:13:40 INFO - JavaScript error: , line 0: NS_ERROR_NOT_AVAILABLE: Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIUrlClassifierDBService.beginUpdate] 10:18:56 INFO - TEST-INFO | started process screentopng 10:18:56 INFO - Xlib: extension "RANDR" missing on display ":0". 10:18:56 INFO - TEST-INFO | screentopng: exit 0 10:18:56 INFO - 3261 INFO TEST-UNEXPECTED-FAIL | toolkit/components/url-classifier/tests/mochitest/test_classified_annotations.html | Test timed out.

This was disabled on linux-ASAN to get run-bu-dir enabled : https://dxr.mozilla.org/mozilla-central/source/toolkit/components/url-classifier/tests/mochitest/chrome.ini#11 This can be tested in the try server and turned back on, once fixed.

Bulk assigning P3 to all open intermittent bugs without a priority set in Firefox components per bug 1298978.