Does not work in Firefox 79+ because SharedArrayBuffer is undefined

**Describe the bug**
The front page says:
> Your browser doesn't support SharedArrayBuffer, thus ffmpeg.wasm cannot execute. Please use latest version of Chromium or any other browser supports SharedArrayBuffer.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://ffmpegwasm.github.io/
3. Scroll down to 'Demo'
4. See quoted error

**Expected behavior**
As `SharedArrayBuffer` is available in the aforementioned versions of Firefox (on certain conditions), I think it is fair we don't fall into the "this only works in Chrome" bane of Web applications made today, especially if the application may in fact work in Firefox and the latter only refuses to provide the object for security reasons: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/79#JavaScript

**Desktop (please complete the following information):**
- OS: Windows 10 x86-64
- Browser Firefox 64-bit
- Version 81.0.1