"WARNING: An error occured while fulfilling user request" in Firefox

[johnnybaloney]

happens in release: 20160618 and 20160515
different message in release: 20151230

I'm following the guide on FitNesse website. Starting FitNesse for the first time:

$ java -jar fitnesse-standalone.jar -p 8080

After opening the home page (http://localhost:8080/) I get the following periodically:

WARNING: An error occured while fulfilling user request [fitnesse.http.HttpException: The request string is malformed and can not be parsed]
java.util.concurrent.ExecutionException: fitnesse.http.HttpException: The request string is malformed and can not be parsed
    at java.util.concurrent.FutureTask.report(FutureTask.java:122)
    at java.util.concurrent.FutureTask.get(FutureTask.java:206)
    at fitnesse.FitNesseExpediter.makeResponse(FitNesseExpediter.java:107)
    at fitnesse.FitNesseExpediter.run(FitNesseExpediter.java:60)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Caused by: fitnesse.http.HttpException: The request string is malformed and can not be parsed
    at fitnesse.http.Request.checkRequestLine(Request.java:159)
    at fitnesse.http.Request.readAndParseRequestLine(Request.java:73)
    at fitnesse.http.Request.parse(Request.java:61)
    at fitnesse.FitNesseExpediter$1.call(FitNesseExpediter.java:104)
    at fitnesse.FitNesseExpediter$1.call(FitNesseExpediter.java:101)
    ... 4 more

This is what I see when I open the page in Firefox 48.0. When I open the page in Chrome 52.0.2743.116 (64-bit) I don't get any warning.

It seems that it gets triggered by hovering over the top menu buttons (Edit, Add, Tools) but I cannot see any network traffic in the developer tools of either browser. However, wireshark shows 400 Bad Request and TCP RST flag sent:

In release 20151230 the behaviour is similar (no warnings when opening in Chrome) but the content of the message is:

Output stream closed unexpectedly: Broken pipe (Stop button pressed?)

[amolenaar]

Hi,

Does this result in functional errors in the application?

I've seen an issue on this subject before. After investigation, it appeared Firefox was opening connections, but not sending any data.

There is a related issue, although I can not find it right now.

[johnnybaloney]

Hi,

I don't think so, it seems to be just noise in the log which I deemed necessary to investigate before actually using the tool. I didn't find anything related in this bug tracker when I encountered the issue for the first time hence this report.

[jediwhale]

Probably related to this but I haven't been able to narrow it down to an actual cause:
https://support.mozilla.org/en-US/kb/how-stop-firefox-making-automatic-connections

[amolenaar]

I can't find any reference either :/

FitNesse will shut down a connection if no request has been received within a reasonable timeframe, 10 seconds by default.