Newly created private tab stuck

[theultramage]

This started happening after updating from FF52 ESR to FF56.0.2 Sometimes, when using the main menu to open a new private tab, possibly when the browser is under load, a stuck inaccessible new tab is created.

• not marked as private (no dotted underline)
• does not gain focus when created
• does not respond to clicks or attempts to close it
• happens both when e10s is turned on and when off

I haven't had the browser console open when the tab was created, but whenver I hover over it, I get

[Private Tab] getTabPrivacyContext() called for already destroyed tab, call stack:
getTabPrivacyContext@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///C:/.../[email protected]!/bootstrap.js:4462:7
isPrivateTab@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///C:/.../[email protected]!/bootstrap.js:4479:24
updateTabTooltip@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///C:/.../[email protected]!/bootstrap.js:2121:49
popupShowingHandler@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///C:/.../[email protected]!/bootstrap.js:2078:4
handleEvent@resource://gre/modules/addons/XPIProvider.jsm -> jar:file:///C:/.../[email protected]!/bootstrap.js:173:38

browser is undefined                 bootstrap.js:4466