Iframe based AJAX call blocked by popup-blocker or opening in new tab

Iframe based AJAX call blocked by popup-blocker. After allowing popups, form submits to new tab instead of hidden iframe.

Issue appeared in TBB 5.5. TBB 5.0.7 (previous stable) works with same code normally.

To reproduce go to http://dmirrgetyojz735v.onion/d/#set-lang:en and try to make post with JS support enabled. Code of submit and AJAX handlers can be found in http://dmirrgetyojz735v.onion/js/tinaib1.js , seek doPostForm func.

Trace is like

WARNING: content window passed to PrivateBrowsingUtils.isWindowPrivate. Use isContentWindowPrivate instead (but only for frame scripts).
pbu_isWindowPrivate@re[/gre/modules/PrivateBrowsingUtils.jsm:25:14](/gre/modules/PrivateBrowsingUtils.jsm:25:14)
nsBrowserAccess.prototype.openURI@chrome://browser/content/browser.js:15194:21

Depending of the exact code, call stack may contain func, which actually submitted form.

Tested on Ubuntu 14.04 x64.

Trac:
Username: sky_kohai