Firefox support (#1606)

This PR adds downloadable firefox packages into emsdk.

The downloading of firefox will be done via the `mozdownload` pip
package, which simplifies the installation a lot.

Installing and activating a Firefox package will set up the
`EMTEST_BROWSER` variable, so running browser tests will then
automatically use the activated browser.

The oldest supported Firefox version by Emscripten is 65. So add that
version, and all ESR versions from 65 upwards, and the moving channels.
This allows people to go back and forth testing different versions of
interest.

.gitignore

@@ -33,3 +33,4 @@ __pycache__
 /mingw
 /spidermonkey
 /binaryen
+/firefox