Add comment about minimum Firefox version required to run parallel test harness.

[juj]

The new parallel Firefox test harness requires Firefox 67 or newer to run.

Firefox 66 and earlier suffer from two bugs:

1. When launching multiple browsers with

[C:\\emsdk\\firefox\\65.0_64bit\\firefox.exe, '-new-instance', '-profile', 'C:\\emsdk\\emscripten\\main\\out\\browser-profile-1', 'http://localhost:5555'

then if a previous instance of Firefox is running, Firefox will interpret the -profile argument as a URL to visit. (It will take that argument as profile path as well). This results in two tabs being opened in each browser instance (beyond the first):

but unfortunately given that the actual test URL is in the background, the test never progresses.

(This issue could be fixed by passing -url parameter, but given this is an ancient browser, and bug 2 also exists, not worth it to complicate code)

2. If a test fails in Firefox 66 or older, and a browser instance is being terminated, it looks like all browser instances get torn down, and not just the one that was hosting the failing test. So this would result in a lot of false positives in test failures.

Bisected that the fix to the above issues occurred somewhere between Firefox 66 and 67, in this bisection range: https://hg-edge.mozilla.org/mozilla-central/pushloghtml?fromchange=4ab143dde4dc3424cfedc74b3648fbf2e47fb7bf&tochange=af29567ecdba5441117b3f01ac2559a3473c3d6a

This PR adds a comment suggesting to set EMTEST_CORES=1 if testing older Firefox versions.

[sbc100]

Should we bump the OLDEST_SUPPORTED_FIREFOX from 65 to 67 to sidestep the issue? Since its only 2 versions it seems unlikely anyone would care.

[sbc100]

It looks like doing so would also allow us to remove a #if MIN_FIREFOX_VERSION < 67 hack in libwebgl.js

[juj]

Should we bump the OLDEST_SUPPORTED_FIREFOX from 65 to 67 to sidestep the issue? Since its only 2 versions it seems unlikely anyone would care.

We could probably then bump to Firefox 68 since that is an ESR release. I have coverage set up for the ESR releases.

[sbc100]

Should we bump the OLDEST_SUPPORTED_FIREFOX from 65 to 67 to sidestep the issue? Since its only 2 versions it seems unlikely anyone would care.

We could probably then bump to Firefox 68 since that is an ESR release. I have coverage set up for the ESR releases.

Yes! Sounds good. How do we find out which are ESR? Maybe we could add a comment alongside OLDEST_SUPPORTED_FIREFOX for how to do that.

[juj]

How do we find out which are ESR?

Searching for "esr/" on this page https://archive.mozilla.org/pub/firefox/releases/ finds ESR releases.