I called for more help. This could be something blocking content or possibly a certificate issue.

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Thanks for the suggestion, FredMcD!

The first time I tried it tabs numbers 1 & 2 loaded all right, but tab number 3 was again without the expected content. Reattempting the safe mode made matters worse in the sense that tab #2 stopped working again.

In case it matters: the data on those tabs updates every few seconds (the tab with previews of live games updates the board position, the tab with active challenges also updates as challenges become available/unavailable)

One user reportedly solved the problem by configuring FF to use something called "squid proxy for htttp / https"

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

I don't understand your latest suggestion. I'm running FF on Windows10. I have not experienced this problem on other sites. But the other sites I frequent don't have content that is supposed to automatically update every few seconds - other than videos. At least not systematically (difficult to tell whether an occasional freeze miht be due to the same issue). So exactly where should I test it?

But, the tech support at that site tested it with several browsers (as did other users). Only FF had problems. One user running FF on Linux figured out that workaround.

Clearing the cache again helped in the sense that now I can have two tabs loading and updating as they should. A third tab still gives problems.

That didn't last long. Only a single working tab again. The cache is stil nearly empty (18MB out of 350MB). Could it be that FF stops waiting for the data to load too soon?

For what it's worth, I doubt this is a local issue. At least I do not know anyone with up-to-date firefox who would not experience it. (All other common browsers work)

Anyone wanting to reproduce it just go to https://online-go.com/ YOU DO NOT NEED TO REGISTER or anything.

You should see a list of ongoing games. Either try refreshing a couple of times, or open "games" or "chat" in a NEW TAB. Sometimes it loads ok, most of the times a subsequent reload or new tab will fail to load the content. (the basic UI will be present, but you will not see any games/chat)

The issue is present for several days now (was present even in the last version of FF before the current one) and the site itself has not changed before the issue appeared.

The console will repeat one of the following two errors, every time the content tries to reload:

The connection to wss://online-go.com/socket.io/?EIO=3&transport=websocket was interrupted while the page was loading.

or

Firefox can’t establish connection with a server wss://online-go.com/socket.io/?EIO=3&transport=websocket.

Apparently they made some changes today at the OGS end. It seems to work now. I first noticed that SHIFT+Refresh started working. It might be prudent to wait for a couple of days, before we can confidently mark this closed. Just in case other FF users still experience some problems.

I'm afraid I don't know what the issue was, and how it was resolved.