Environment:
Operating System: Ubuntu 20.4 LTS x64
Firefox version:Firefox Nightly 99.0a1 (2022-02-28) (64-bit)

Preconditions:
ETP set to STANDARD
Clean profile

Steps to reproduce:

1. Navigate to: https://www.labelferme.fr/carte-de-fidelite
2. Observe the result.

Expected Behavior:
The contact details form is shown.

Actual Behavior:
The contact details form is not shown.

Notes:

• Not reproducible with ETP turned off.
• Works as expected using Chrome
• Screenshot attached

Raul, Paul, Ben and I aren't able to reproduce this in standard mode; it breaks in strict mode, but in standard mode the contact details appear for us, there is just a slight delay before it loads compared to ETP being off (which is probably because we de-prioritize loading tracker-related requests with ETP on). Is that maybe what you ran into?

Tom, after leaving the tab open for about 10-15 minutes, for me, the form still does not load with ETP set to STANDARD, on all versions of Firefox for desktop and mobile Android>

Tested with:

Browser / Version: Firefox Release 97.0.1 (64-bit)/ Firefox Nightly 99.0a1 (2022-03-02) (64-bit)/Firefox Beta 98.0b9 (64-bit)
Operating System: Windows 10 PRO x64

Tested with:

Browser / Version: Firefox Release 97.2.0 (2015863827-🦎97.0.1-20220216172458🦎)/ Firefox Nightly 99.0a1 (2015866539 -🦎99.0a1-20220302065857🦎)/ Firefox Beta 98.0.0-beta.4 (2015865235-🦎98.0-20220222185824🦎)/
Operating System: Samsung A51 (Android 11) -1080 × 2400 pixels 20:9 aspect ratio (~405 ppi density)

I get the same behavior as you, Tom, with ETP - Strict the form does not load, but with ETP - Standard it loads both on desktop and mobile.
https://prnt.sc/prQJeVoOlfiV

Tested with:
Browser / Version: Firefox Nightly 99.0a1 (2022-03-03), Firefox 100, Firefox Nightly 99.0a1 (🦎 99.0a1-20220302065857)
Operating System: Windows 10 Pro, Google Pixel 5 (Android 12) - 1080 x 2340 pixels, 19.5:9 ratio (~432 ppi density), Samsung Galaxy S8 (Android 9) - 1440 x 2960 pixels, 18.5:9 ratio (~570 ppi density)

The missing form is loaded via third-party iframe hosted at https://labelferme.us2.list-manage.com/subscribe, so the best we can probably do here is to present the user with an intermediate opt-in for the iframe, letting them load the tracked content.