Adding any parts on Pcpartpicker results in "Unknown error occured" using latest Waterfox 56.2.6.
I have to use old Firefox for Pcpartpicker instead for now.
To Reproduce
Expected behavior
pcpartpicker website functioning normally.
Desktop (please complete the following information):
Add any CPU
Done:

Waterfox 56.2.6 here, FreeBSD-CURRENT.
"Unknown error occured"
Is the issue reproducible with Waterfox in safe mode?
Reproducible with a new profile?
In safe mode the same "unknown error occurred" message appears instead of adding the part.
In another profile(newly created) the website works normally. Strange.
extension isolation tests:
New profile(no extensions: website works
Default profile with Noscript disabled: pcpartpicker does not work
With right to Click disabled: does not work
With Request Policy Continued disabled: does not work
HTTP nowhere disabled; does not work
Ublock origin disabled:does not work
Greasemonkey disabled:does not work
Multiple tab handler Disabled :does not work
Textarea cache disabled:does not work
Stylus disabled: does not work
Down them all and Down them all anticontainer disabled:does not work
TabMixPlus disabled: does not work
Developer tools shows this text is received before the error:
this page is not displayed in the browser.
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<head>
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<meta name="robots" content="NONE,NOARCHIVE">
<title>403 Forbidden</title>
<style type="text/css">
html * { padding:0; margin:0; }
body * { padding:10px 20px; }
body * * { padding:0; }
body { font:small sans-serif; background:#eee; }
body>div { border-bottom:1px solid #ddd; }
h1 { font-weight:normal; margin-bottom:.4em; }
h1 span { font-size:60%; color:#666; font-weight:normal; }
#info { background:#f6f6f6; }
#info ul { margin: 0.5em 4em; }
#info p, #summary p { padding-top:10px; }
#summary { background: #ffc; }
#explanation { background:#eee; border-bottom: 0px none; }
</style>
</head>
<body>
<div id="summary">
<h1>Forbidden <span>(403)</span></h1>
<p>CSRF verification failed. Request aborted.</p>
<p>You are seeing this message because this HTTPS site requires a 'Referer header' to be sent by your Web browser, but none was sent. This header is required for security reasons, to ensure that your browser is not being hijacked by third parties.</p>
<p>If you have configured your browser to disable 'Referer' headers, please re-enable them, at least for this site, or for HTTPS connections, or for 'same-origin' requests.</p>
</div>
<div id="explanation">
<p><small>More information is available with DEBUG=True.</small></p>
</div>
</body>
</html>
After looking into it, i've tried changing
network.http.sendRefererHeader from 1 to 2
and pcpartpicker.com started working normally.
Closing this topic.