I remember I had that issue once, too, and I believe I tracked it down to the following.
{"isTrusted":true}
was the body of a request that was printed by my ErrorHandler that caught failed requests. The cause for the failed request was a CORS issue. I had a wildcard set for allowed origins in my Tomcat's web.xml for testing purposes:
<init-param>
<param-name>cors.allowed.origins</param-name>
<param-value>*</param-value>
</init-param>
It turned out that Firefox, at least in that particular version I used, didn't like wildcards and, thus, resulted in a failed request although the preflight had succeeded. After setting the origin to a qualified name, everything worked fine. And just like you, I never had those issues in Chrome.
See this SO question, too
Hope that helps to track down your issue.
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