I have no idea. I use Chrome normally, but I opened Firefox and downloaded the update to 66, then opened S+ [because of course I use the newer StrokesPlus.net now :) ] and acSendKeys("^t") opens a new tab for me fine.
Are you using a non-English keyboard layout? That's one thought.
You can see if this has a different resultacSendControlDown()
acSendKeyDown(84)
acSendKeyUp(84)
acSendControlUp()