STR: 1. Turn on privacy.resistFingerprinting & restart. 2. Go to youtube.com 3. Open a video in a new tab. 4. Click on a different video in the new tab. 5. Navigate back. The URL changes when going back but the video does not change and instead starts playing from the beginning. This is a frequent but intermittent failure so it may be necessary to repeat steps 3-5. Refreshing the page loads the correct video corresponding with the URL. Regression range: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=1742b1bdadd13a02df95ca690bea9cc42ff40c91&tochange=b996f14177a63f5f27d5222e8f1f86e9155e98da Regressed by Bug 1217238. privacy.reduceTimerPrecision = false has no effect with resistFingerprinting enabled by design. privacy.resistFingerprinting.reduceTimerPrecision.microseconds = 1 does not make any difference. It can be reproduced with privacy.resistfingerprinting = false by setting reduceTimerPrecision.microseconds = 1000000. I made a build without the call to ReduceTimePrecisionAsMSecs() in Event.cpp and the problem no longer occurred with resistFingerprinting enabled.

tjr, please take a look.

Wow, thank you for the very detailed report! We are a bit backlogged on Resist Fingerprinting efforts right now unfortunately, but this is really helpful and we're hoping to work on RFP breakage soon. One question, you said: >reduceTimerPrecision.microseconds = 1000000 That's 1 whole second; normally resist fingerprinting operates at 100ms (or 100000 for the pref). Since you encountered this with just RF I'll presume 100ms also triggers the behavior.

I used one second precision as an extreme example to increase likelihood of occurrence. I can confirm it happens often at 100ms (100000) and down to 30ms although less frequently. I have not encountered it at 10ms in my testing. Only the new youtube design is affected.