Thanks for the feedback. Are using Firefox on Linux ?
I have added a fix for Firefox on Linux, can you update to the latest version and test again?

Edit:
I'm done with the analysis and it is a bug for Firefox-on-Windows with apparently no workarounds :-/
I'll file a bug report to Firefox.
Till then, I don't see any other alternative approach while sticking to double-right-click-on-page gesture for closing tab.