This isn't really about the suggestions API, but about validating URL strings. I made a testcase based on https://searchfox.org/mozilla-central/rev/2c3d657cbba5484ccac44443c4417baed7b5fafb/testing/web-platform/tests/conformance-checkers/tools/url.py and it looks like we're still allowing a number of things that shouldn't be allowed (missing // after the scheme for special schemes, non-digit in port, whitespace in various pieces, …).
Bug 561586 Comment 5 Edit History
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This isn't really about the suggestions UI, but about validating URL strings. I made a testcase based on https://searchfox.org/mozilla-central/rev/2c3d657cbba5484ccac44443c4417baed7b5fafb/testing/web-platform/tests/conformance-checkers/tools/url.py and it looks like we're still allowing a number of things that shouldn't be allowed (missing // after the scheme for special schemes, non-digit in port, whitespace in various pieces, …).