Upstream bug https://bugzilla.mozilla.org/show_bug.cgi?id=366945 is
showing as "VERIFIED FIXED". Tested OK in Ubuntu 18.04 and Firefox 61.
Marking as fixed to close.
** Bug watch added: Mozilla Bugzilla #366945
https://bugzilla.mozilla.org/show_bug.cgi?id=366945
** Changed in: firefox (Ubuntu)
This will be disabled by default in Firefox 57, currently scheduled for
release on 14th November 2017. See
http://www.phoronix.com/scan.php?page=news_item&px=Firefox-Middle-Click-
Bug for more details.
** Changed in: firefox (Ubuntu)
Status: New => Fix Committed
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