Here's the stack: permitUnload@chrome://global/content/bindings/remote-browser.xml:357:13 _beginRemoveTab@chrome://browser/content/tabbrowser.xml:2705:46 removeTab@chrome://browser/content/tabbrowser.xml:2632:18 closeTab@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/tabs/utils.js:168:12 close@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/tabs/tab-firefox.js:163:5 extendTab/tab.stopAndClose@resource://gre/modules/commonjs/toolkit/loader.js -> resource://jid0-rvyt2rgwfm8q5ywxixahyaeo5qg-at-jetpack/lib/tabsManager.js:178:3 manageUniqTab@resource://gre/modules/commonjs/toolkit/loader.js -> resource://jid0-rvyt2rgwfm8q5ywxixahyaeo5qg-at-jetpack/lib/duplicateTabManager.js:82:6 filterDuplicateTabs@resource://gre/modules/commonjs/toolkit/loader.js -> resource://jid0-rvyt2rgwfm8q5ywxixahyaeo5qg-at-jetpack/lib/duplicateTabManager.js:102:8 searchDuplicateTabs@resource://gre/modules/commonjs/toolkit/loader.js -> resource://jid0-rvyt2rgwfm8q5ywxixahyaeo5qg-at-jetpack/lib/duplicateTabManager.js:109:2 closeDuplicateTabs@resource://gre/modules/commonjs/toolkit/loader.js -> resource://jid0-rvyt2rgwfm8q5ywxixahyaeo5qg-at-jetpack/lib/duplicateTabManager.js:128:38 initialize/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://jid0-rvyt2rgwfm8q5ywxixahyaeo5qg-at-jetpack/lib/panelManager.js:52:52 emitOnObject@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:110:7 emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:86:38 portEmit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/content/sandbox.js:359:7 emitOnObject@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:110:7 emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:86:38 onContentEvent/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/content/sandbox.js:400:5 delay/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/lang/functional/concurrent.js:39:20 notify@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/timers.js:40:9

I'm afraid what's happening here is that we're spinning a nested event loop, and then some native code somewhere is causing the flush. Nothing actionable in here, I'm afraid.