Does the page load if you press F5?
Here's an update on my experience with the problem.
The problem only occurs after I leave the Manjaro forums by loading another domain in the same Firefox tab and than back-page in the same tab to the Manjaro forums. (The problem also occurs when I start up Firefox with the Manjaro forums in a saved tab).
Pressing F5 always loads the page, but as soon as I leave the domain and then return, the problem always occurs again immediately.
When I create a new Firefox profile, the problem does not recur immediately, but it always returns eventually. It returns even if I have not installed any add-ons in the new profile. So, in my experience with this problem, it has nothing at all to do with add-ons.
Clearing site data (see my instructions in my earlier post) works as well as (and is easier than) creating a new Firefox profile. The problem does not recur immediately, but it always returns eventually (and no sooner than it does with a new profile).
So I recommend clearing site data. The forums should then be trouble-free for at least a while.
By the way, I have used Firefox Web Developer Tools to monitor network traffic and the values of variables in local storage. I don't see any anomalous behavior with the forums, even when the error occurs.
However, I did find it interesting that, after clearing site data, the app does not begin to use local storage again immediately. Eventually, it does begin to use local storage, but until then, the forums behave normally. Oddly, the problem doesn't always recur immediately upon the initialization of local storage, but it can take a few hours longer.