FF 38.0.1, Linux

Steps to reproduce:

• Copy text including a whitespace, e.g. " https://phabricator.wikimedia.org/tag/fundraising_tech_backlog/".
• Insert a new external link.
• Set the external URL to your pasted text, including the whitespace.

The URL field will have a red error border, and the Insert button is disabled.

More polite behavior would be to ignore the whitespace when testing for URL validity, and strip it if Insert is pressed.