The changes in Bug 1583696 broke the vertical alignment of the prompt text. I've attached screenshots.

Tested on Linux and Win10, latest Nightly, clean profile.

Mozregression confirms this: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=bcabae9c2fed5d2a90657f63161319e9fb08024b&tochange=5ed5e8a6bbfb750504c240ef149149308f50a031

[Tracking Requested - why for this release]:
as-yet unshipped user-visible regression

.tabmodalprompt-topContainer { align-content: center; } might help?

See: https://bugzilla.mozilla.org/show_bug.cgi?id=1583696#c38 for more context.

https://hg.mozilla.org/mozilla-central/rev/8968a5912c66

Comment on attachment 9188329 [details]
Bug 1677725 - Restore older alignment behaviours on content tab prompts.

Beta/Release Uplift Approval Request

• User impact if declined: Visual regression in JS alert/confirm/prompt UI
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: Run alert("Hello world") in the console or use https://javascript.info/alert-prompt-confirm

It would also be nice to test edge cases (e.g long text or long text with lots of spaces in between) as well, to see if nothing regressed from this patch.

• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): small styling change, easy to backout
• String changes made/needed: No

Comment on attachment 9188329 [details]
Bug 1677725 - Restore older alignment behaviours on content tab prompts.

Approved for 84.0b3.

https://hg.mozilla.org/releases/mozilla-beta/rev/0d7dbfa4aea7

I managed to reproduce the issue on an older version of Firefox using Windows 10 x64.
I retested everything using latest Nightly and Firefox 84.0b3 (treeherder) on Windows 10 x64, Ubuntu 18.04 x64 and macOS 10.15. The issue is not reproducing anymore.