It would be nice to provide a self-contained testcase working online.
Oh I surfed in Bugzilla and I found a bug that might be the cause. Bug 833774 - The displayed HTTPS response status comes from proxy, not from web server https://bugzilla.mozilla.org/show_bug.cgi?id=833774 Since in the debugger of Firefox when problem happens, it shows "200 Connection established", but when I use Fidder to capture the traffic using HTTPS tunnel (or proxy), it returned 200OK and I can upload the file.
I found another related bug Bug 1139479 - SSL Error during file upload by POST in HTTPS https://bugzilla.mozilla.org/show_bug.cgi?id=1139479 I have tested the filesize limits is exactly 7557 characters, or 7KB.
As you're able to reproduce it, could you download and run the tool mozregression to find a possible regression range, please. See http://mozilla.github.io/mozregression/documentation/usage.html for details about istall and usage. If it worked in FF36, run "mozregression --good-release 36" and copy here the pushlog (no need to bisect more).
No testcase and the needinfo? request has gone 6 months without response. I'm going to close this out as incomplete. John, if you can still reproduce this issue and want to take another stab at moving it forward, feel free to needinfo? me and we can do so.
Status: UNCONFIRMED → RESOLVED
Last Resolved: 3 years ago
Resolution: --- → INCOMPLETE
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