(In reply to Lars T Hansen [:lth] from comment #2)
I misremembered how the threads ops are configured. They are always present, at the moment, there is no moz.configure flag. But they may only be used if the memory is shared (we check this during opcode reading), and a shared memory can only be constructed if the correct pref if flipped in about:config. However, bug 1619196 changes that logic because it allows the thread ops to be used always; i'm not sure what the fallout is.
Bug 1648755 introduces proper gating for thread ops on the pref. Then bug 1619196 allows thread ops to be used on non-shared memories. These changes compose properly, ie, a thread op on any kind of memory will be rejected if the pref is off.
With this change, simply flipping the pref will enable wasm thread ops, meaning that wasm thread ops will ride along with resab.
IMO the resab intent-to-ship was not real clear about wasm thread ops, and I will follow up to that with a clarification. A completely separate intent-to-ship is not really appropriate since wasm thread ops ship along with resab.