iTerm opens instead of default web browser when links clicked in Apple Mail
Rebooting solved this one. After the reboot, the list of browsers under Apple > System Preferences > General > Default web browser was restored, and clicking web links in Mail.app resumed opening in my default browser.

Interestingly, ~/Library/Preferences/com.apple.LaunchServices.plist, which was empty before the reboot, remained empty afterwards, so I'm guessing the list of browsers isn't stored there. (Thought it might be stored there because of this post). (The file /Library/Preferences/com.apple.LaunchServices.plist doesn't exist, either)