• turn on linter (bug 1511152)
• aria-hidden=true for content behind modal
• keep tabbing focus inside modal
• automatically focus email input
• role=dialog on modal with aria-labelledby=<id of heading>
• allow keyboard escape from dialog (bug 1548267)

I'm also happy to look at it again in more detail once things on the list are addressed.

In the meanwhile, just like with dialog semantics I would also generally pay attention to semantics of other widgets/controls/etc within the new UI and text alternatives for non-textual bits where needed.

FYI, I looked at colour contrast and there are no issues with onboarding dialog or the cards that show up after it's dismissed.

https://hg.mozilla.org/mozilla-central/rev/690025a0dcd5

I have verified this issue with the latest Firefox Nightly (68.0a1 Build ID - 20190509214305) installed, on Windows 10 x64, Arch Linux and Mac 10.14.4. Now, the "First Run" page elements are respecting the specs from comment 0.

Marking verified as per bug 1550098 Comment 13 for bugs that were status-firefox68=verified