Sorry for the inaccurate shorthand; maybe that should read that SpiderMonkey supports it. Eric Faust of SpiderMonkey is a co-champion of the proposal, and spoke against implicit PTC at the March 2016 TC39 meeting. It's hard to get much stronger in support of a proposal than being a champion, and Eric works for Mozilla. From that discussion, it also sounded like there was support from the Mozilla devtools team as well.
EDIT: How do you like the new wording "For these reasons, the V8 team strongly support denoting proper tail calls by special syntax. There is a pending TC39 proposal called syntactic tail calls to specify this behavior, co-championed by committee members from Mozilla and Microsoft." ?
I'm interested in getting everyone's point of view. We've been discussing pros and cons at https://github.com/tc39/proposal-ptc-syntax/issues and https://github.com/tc39/ecma262/issues/535 , and it'd be great to have your input, including the overturning-prior-consensus issues you raised in committee and anything else that comes to mind.
EDIT: How do you like the new wording "For these reasons, the V8 team strongly support denoting proper tail calls by special syntax. There is a pending TC39 proposal called syntactic tail calls to specify this behavior, co-championed by committee members from Mozilla and Microsoft." ?