Thunderbird: Open Message URL

userChrome.js

// Install https://github.com/jikamens/userChromeJS and save this as chrome/userChrome.js in your TB profile directory

window.addEventListener('keydown', function (event) {
	if (event.metaKey && event.ctrlKey && event.key == 'o') {
		var loadFirstURLInBody = function (body) {
			var parser = new DOMParser();
			var doc = parser.parseFromString(body, "text/html");
			var links = doc.getElementsByTagName('a');
			if (links.length) {
				let uri = Services.io.newURI(links[0].href, null, null);
				Components.classes["@mozilla.org/uriloader/external-protocol-service;1"]
					.getService(Components.interfaces.nsIExternalProtocolService)
					.loadURI(uri)
			}
		}

		var msgHdr = gFolderDisplay.selectedMessage;
		var folder = msgHdr.folder;
		if (folder.name == 'Zotero Forums') {
			FeedMessageHandler.loadWebPage(msgHdr, { browser: true });
		}
		else {
			MsgHdrToMimeMessage(msgHdr, null, function (msgHdr, msg) {
				for (var i=0; i<msg.parts.length; i++) {
					var part = msg.parts[i];

					if (part.contentType == 'text/plain') {
						let matches = part.body.match(/https?:\/\/[^\s$]+/);
						if (matches) {
							let uri = Services.io.newURI(matches[0], null, null);
							Components.classes["@mozilla.org/uriloader/external-protocol-service;1"]
								.getService(Components.interfaces.nsIExternalProtocolService)
								.loadURI(uri)
							return;
						}
					}
					else if (part.contentType == 'text/html') {
						loadFirstURLInBody(part.body);
						return;
					}
					else if (part.contentType == 'multipart/alternative') {
						for (var j=0; j<part.parts.length; j++) {
							var subpart = part.parts[j];
							if (subpart.contentType == 'text/html') {
								loadFirstURLInBody(subpart.body);
								return;
							}
						}
					}
				}
			}, true);
		}
	}
});