That is likely a problem with Total Cookie Protection that isolates third-party cookies, so those cookies are only exposed on that settings page and not on other pages.

You can check for issues with Total Cookie Protection.

• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/introducing-total-cookie-protection-standard-mode

The requests are redirected (302 = Temporary Redirect). I don't know whether that is a factor.

First redirect:

Referer = account.microsoft.com Host = api.choice.microsoft.com Cookie set on: .microsoft.com Effect: success

Second redirect:

Referer = account.microsoft.com Host = api.choice.msn.com [cross-site from referer] Cookie set on: .msn.com Effect: failure

Third redirect:

Referer = account.microsoft.com Host = api.choice.bing.com [cross-site from referer] Cookie set on: .bing.com Effect: failure

Fourth redirect:

Referer = account.microsoft.com Host = api.choice.live.com [cross-site from referer] Cookie set on: .live.com Effect: failure

I can't help thinking that it's the cross-site nature of the request that causes the problem. Even though you have samesite=none in the Set-Cookie header.

Could you install the MozRegression tool and see which build caused it to stop working? Your security software may balk at some unsigned development builds, but you may be able to at least figure out the major version in which it stopped working, even if you can't run the development builds. More info:

https://mozilla.github.io/mozregression/

We've always done the redirect, and it worked a few years ago. I tried the regression tool, and narrowed the range down to sometime in mid 2020 (2020-08-06 bad, 2020-03-15 good), but ran into a bunch of weird crashes around May/June that made testing hard. I'll keep working with it when I have the time.

Can you think of something that changed in that timeframe?

Hmm, it's kind of puzzling to get a regression range in 2020 if Firefox 115 ESR is working. Unless there was a change that was made, discovered to cause problems, and then later backed out. ??

(As for my memory of that time, I had some other things on my mind...)

My coworker reported it working on the corporate network in 115, but I'm unable to reproduce this behavior with the regression tool. Whenever I test with the tool, the failures start mid-2020. I verified this on my work machine and on a personal laptop. Unfortunately, also in mid-2020 FF just starts crashing constantly, so I can't track it down further than that. This also occurs on my work machine and personal laptop. The best I can get is "somewhere between March and September".

I'm trying to get my coworker to also test, but he has other things to do and because of market share, this isn't a super high priority. I care, but it's hard to get other people on board.

Thank you for testing as far as you could. I wonder whether your coworker has a modified setting that allows it to work in their installation. If they want to compare vanilla settings -- assuming Group Policy doesn't reinject a modification -- they could run a quick comparison using the following steps.

New Profile Test

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Test2024, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Test2024 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

Do the cookies get set (and persist) in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2024 will remain available for future testing.)

I asked my coworker, and he confirms it doesn't work if he makes a new profile. So it looks like he did something to the original profile and it is broken in 115.