There are still 14 issues blocking the MVP version of enabling Shared Array Buffers in Firefox by default. https://bugzilla.mozilla.org/show_bug.cgi?id=1477743 And two of those are "design" tasks that don't even count the implementation work yet.
You can enable the feature manually by toggling javascript.options.shared_memory in about:config.
Summary:. Shared Array buffers, used by multithreaded webassembly, can be used in combination with a spectre attack to read the memory of the Firefox renderer process.
If that renderer has also loaded your Google/Facebook cookies, that's a really big problem, since the attacker can now read all your email or post rude things on your mum's wall.
Chrome has solved this with site isolation - every domain gets its own renderer process.
It's a lot of work to do the same in Firefox. And the performance and memory impact of site isolation is pretty big.
