User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:80.0) Gecko/20100101 Firefox/80.0

Steps to reproduce:

1.Open url(https://nmsg.nicovideo.jp/api.json/thread?version=20090904&thread=1584093666&res_from=-1000)
JSON Viewer is not working too!
2. While you are selecting this tab, open customize on tool bar then hit "complete" button
3. The tab's contents will disappear and you will see cycling icon, and Firefox's tool bar icons will not work at all.

Actual results:

You can not control anything if this happen unless you will close the tab.

Expected results:

Content of tab must not disappear and should toolbar icon work normally.
And JSON Viewer should work too.

Extension icons are still working yet. Maybe thanks to separated extension process?

1st report is bad. It's unreproducable, but I know other way to reproduce this, so I will write here.
But not working JSON Viewer is consistent bug yet, so I will file it as another bug.

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:80.0) Gecko/20100101 Firefox/80.0

Steps to reproduce:

1.Open url(https://www.nicovideo.jp/ranking?header)
2. Right click and open context menu, click view source
3. You can not see source forever how much time you wait.
4.Close source tab.
5. The tab's contents will disappear and you will see cycling icon, and Firefox's tool bar icons (Forward, Backward, Home Buttons) will not work at all. Extension icons are working.

Actual results:

You can not control anything if this happen unless you will close the (bugged) tab.

Expected results:

Content of tab must not disappear and should toolbar icon work normally.
Even in new profile you will see this bug too. If it is Chrome, the bug won't happen.

Comment on attachment 9174535 [details]
bandicam 2020-09-09 07-31-10-483.avi

this vid is invalid now.
please igonore.
see post #3 please.

new video. this is now reproducible even new profile too.
source will never shown how much you wait.
and after you close view-source: tab the original tab will become rotating icon and Firefox's forward icon, backward icon, home icon will be uncontrollable.

I can reproduce the issue on Nightly82.0a1 Windows10 with STR of comment #2.

Workaround: using external editor for view source.

i.e.
user_pref("view_source.editor.external", true);
user_pref("view_source.editor.path", "C:\windows\system32\notepad.exe");

profiler log: https://share.firefox.dev/2FchCr8

At least, view source hangs up, regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=67d1049b0bf9&tochange=978002c0b0ad

I can reproduce that View Source hangs.

(In reply to roflsunriz from comment #0)

JSON Viewer is not working too!

JSON Viewer is shown only for valid JSON content types. See this condition:
https://searchfox.org/mozilla-central/rev/b2716c233e9b4398fc5923cbe150e7f83c7c6c5b/devtools/client/jsonview/Sniffer.jsm#53

The page is served with 'text/json', which doesn't seem to be valid JSON type, or is it?

Honza

(In reply to Jan Honza Odvarko [:Honza] (always need-info? me) from comment #11)

(In reply to roflsunriz from comment #0)

JSON Viewer is not working too!

JSON Viewer is shown only for valid JSON content types. See this condition:
https://searchfox.org/mozilla-central/rev/b2716c233e9b4398fc5923cbe150e7f83c7c6c5b/devtools/client/jsonview/Sniffer.jsm#53

The page is served with 'text/json', which doesn't seem to be valid JSON type, or is it?

Honza

Seems yes. I will mark it as invalid which I have already filed bug.(https://bugzilla.mozilla.org/show_bug.cgi?id=1663804)
Anyway, what should I do to use JSON viewer even invalid MIME type? I want use JSON Viewer.
Of course I am not administrator of this server and I can not change its sending header.
I'm running proxy server but I am still not aware to manipulate its header info.
Or changing Firefox's setting could help this to use JSON Viewer?