If the WebConsole is open, the problem does not occur.

Steps to reproduce:
0. DO NOT open any devtool to reproduce the issue.

1. Open https://photos.google.com/ and login
2. Click Upload button at the top-right of page
   And choose "Computer" so that file picker will pop up
3. Select an image from the file picker

OR
0. DO NOT open any devtool to reproduce the issue.

1. Open https://photos.google.com/ and login
2. Drag an image from file explorer and drop it to page

Acrual Results:
"Can’t upload item". Fails to upload.

Webconsole shows:
Failed to load ‘https://photos.google.com/_/upload/uploadmedia/interactive?authuser=0&upload_id=AEnB2Uoql75oiAHH3ihieb89ZCnpJQAFTSvZC0HvyDRLAFzfZbd4VNthGefrvXsWwZnefeg7WvwS2hjJo0Hp51aT4MzoVd7Jxg&upload_protocol=resumable’. A ServiceWorker passed a promise to FetchEvent.respondWith() that rejected with ‘TypeError: NetworkError when attempting to fetch resource.’.

Expected Results:
Successfully uploaded

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=6f9b74f66da38a0896440350509d4aaf03324e77&tochange=a8e46a5f7f165191a0203f56c6caffb6d61441b2

[Tracking Requested - why for this release]: Cannot upload image to Google Phots. Something (active docShell?) is broken of without open devtool.

The regression window in Comment 0 seems to be erroneous (it's the error message, not a link to a given pushlog)
Would you still have it around?

(In reply to Nicolas Chevobbe [:nchevobbe] from comment #2)

The regression window in Comment 0 seems to be erroneous (it's the error message, not a link to a given pushlog)
Would you still have it around?

Sorry, copy & paste miss, I will correct.
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=6f9b74f66da38a0896440350509d4aaf03324e77&tochange=a8e46a5f7f165191a0203f56c6caffb6d61441b2

Wait, the regression window is incorrect.
If the site had been opened in two tabs(even if one of them was closed), the problem seems to occur.

Corrected ression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=720c1e5a8dd3f5bda4ae32137d1c624a1ad55301&tochange=be9a6289486a6f366e431782b84a0c0633f8fec2

We suspect this to have the same reason as bug 1622451.

These are current, open bugs with a Severity of critical. The Severity of these bugs is being changed to S2 to be consistent with the May 4 2020 Severity definitions.

Please let Release Management know if these bugs are still S2.

Alice0775 White, can you please retest now that bug 1622451 is fixed?

(In reply to Julien Cristau [:jcristau] from comment #8)

Alice0775 White, can you please retest now that bug 1622451 is fixed?

I can reproduce the issue on Nightly78.0a1(20200529095426) Windows10.
And I verified that the issue was fixed. The issue was not reproduced on Nightly78.0a1(20200531214211) Windows10.

Thank you!