Completed mozregression tests, the last good build was b9a0d600 in autoland.
The final autoland bissection was e0555201 - 4ec5f59a; of which builds, only b9a0d600 was good.
mozregression's last logs:
Narrowed inbound regression window from [b9a0d600, 591fefb0] (3 builds) to [b9a0d600, a672ddc3] (2 builds) (~1 steps left)
Found commit message: Bug 1503022 - Refactor window and toolbar color handling and make the Dark and Light themes honor the Windows 10 accent color setting. r=ntim Differential Revision: https://phabricator.services.mozilla.com/D10736
(Never used mozregression before so let me know if you need more details from the log.)
I think the commit message is quite interesting there, as it does seem to have some relationship to the issues which are being observed. After reading that message, I've been trying different combinations of Windows light/dark theme and Firefox theme, and other Windows 10 personalisation/colour settings, but am still seeing the same effects in Firefox on my impacted machines. For reference: I'm using Windows' dark theme, with Firefox's dark theme, but from all my testing so far it does not appear as though changing either Windows or Firefox theme as an impact on the behaviour.
It is also of note that some of the builds presented by mozregression seemed to exhibit different variations of the problematic behaviour, although it's hard to verify this. For example, in some of the builds, when the "white tab strip issue" appeared, it would visibly transition to and from white on activating/deactivating the window. In the later builds and 65 final, this doesn't occur; there's no transition.
It is of further note that I have just booted up a laptop I haven't used in a long-time running Windows 10 16299 (1703), and successfully updated Firefox from 59 up to 64, and then 65, via auto-update; 65 is running normally with none of the issues I'm seeing on the other machines. This laptop runs 32-bit W10 + Firefox, the others are 64-bit.
I'm going to continue testing different combinations of settings, in Windows and Firefox, and will keep reporting back here. My notable next step is to get this laptop up to Windows 10 1803, and then 1809 if it's offered, and see if the issues do reappear then. Then, once I've done that test, I'm also going to apply my titlebar colour registry edits to this laptop (which is clean, and I've never hacked about it with before) to see if I can invoke the issue.
I am aware it's plausible my registry hacking could somehow interfere with DWM, but as I've said before, I've removed the registry keys I was using and it's not had any effect. It's just a change of a couple of DWM keys to enable black dark-theme titlebars while retaining a regular Windows accent colour over the rest of the UI.
That's it for me from now. As I've said, I'm going to do some more testing, and will also be spinning up a VM to try even more combinations if I get a chance.
Please let me know of any further next steps, or if you need further mozregression log output, or if there's anything else I should try.