Firefox e2e tests flakey

[tunetheweb]

Looks in Firefox Nightly FCP is often quite slow in our e2e tests.

For example this run has this when I turn on debugging:

[0-1] reports the correct value after the first paint {
[0-1]   name: 'FCP',
[0-1]   value: 53[15](https://github.com/GoogleChrome/web-vitals/actions/runs/8923549678/job/24508034241#step:6:16),
[0-1]   rating: 'poor',
[0-1]   delta: 5315,
[0-1]   entries: [
[0-1]     {
[0-1]       name: 'first-contentful-paint',
[0-1]       entryType: 'paint',
[0-1]       startTime: 5315,
[0-1]       duration: 0
[0-1]     }
[0-1]   ],
[0-1]   id: 'v4-[17](https://github.com/GoogleChrome/web-vitals/actions/runs/8923549678/job/24508034241#step:6:18)14651853858-9340183650873',
[0-1]   navigationType: 'navigate'
[0-1] }

A rerun often solves it as you can see later in that log:

[0-1] reports the correct value after the first paint {
[0-1] name: 'FCP',
[0-1] value: 72,
[0-1] rating: 'good',
[0-1] delta: 72,
[0-1] entries: [
[0-1] {
[0-1] name: 'first-contentful-paint',
[0-1] entryType: 'paint',
[0-1] startTime: 72,
[0-1] duration: 0
[0-1] }
[0-1] ],
[0-1] id: 'v4-1714651859617-3386553824299',
[0-1] navigationType: 'navigate'
[0-1] }

But it doesn't seem to solve it for these tests which have been continually failing:
- FCP - 'reports if the page is restored from bfcache'
- LCP - 'reports the correct value on hidden (reportAllChanges === false)'

I hoped it was just some nightly flakiness but doesn't seem to be getting resolved so we might want to comment these out for now for Firefox.

I can't reproduce this on MacOS, nor on linux (Chromebox) so it only seems to happen in GitHub Actions. Possibly related to there not being a display there?

[tunetheweb]

Raised a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1894695

[tunetheweb]

So #476 resolved a lot of the cases for now but this one is still susceptible to failing intermittently due to the Firefox slowness in GitHub Actions:

web-vitals/test/e2e/onLCP-test.js

Lines 715 to 718 in fd8b0c3

	const assertFullReportsAreCorrect = (beacons) => {
	const [lcp1, lcp2] = beacons;
	assert(lcp1.value < 500); // Less than the image load delay.

A rerun often fixes it but might take a few attempts. If it gets really annoying we can consider skipping this last line out for Firefox too.

[tunetheweb]

Either way leaving this open to remind us to remove those workarounds once the real fix is released.