The border corners for error message bar are not rounded

[Bianca-O]

Build:
Stage

Affected platforms:
Windows 10 64-bit
macOS 10.14
Android 11
iOS 14.6

Preconditions:
The user has an active MPP subscription.
No email domain added.

Browsers tested on:
Firefox

Steps to reproduce:

1. Navigate to https://stage.fxprivaterelay.nonprod.cloudops.mozgcp.net/ and log in using an existing account.
2. In the "Search your new domain" field type a long email domain and click "Get it" to force an error message.
3. Observe the error message bar.

Actual result:
The corners of the message bar are not rounded.

Expected result:
The corners of the message bar are rounded.

See Figma for reference:

[AOiegas]

Verified this issue on stage, on both Windows 10 and Android 9. The error message bar appears in the correct shape.
To be noted:

• The issue #998 appeared;
• Comparing with the Figma references, there is a length of space missing from beneath the error message bar;