Thanks for the link to the actual Launchpad thread(s) your reply. Mozilla traced the issue for Firefox 39.0-build2-0ubuntu0-0.12.04.1 to a mis-compile of the i386 version, meaning an unforeseen issue with the GNU Compiler Collection on 12.04-LTS. Hopefully they can fix the GCC bug(s) and get 39.0-build6-0ubuntu0.12.04.1 out to us.
Addendum: Don't know whether the GCC issue is a zero-day bug as of 13 July 2015, but no mis-compiles have been reported on other Releases.
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