fx-compat: Add message to Add-ons pane when no plugins installed

We'll want to update the rest of the instances of "extensions" or
"add-ons" to say "plugins" (unless we think we might support themes, in
which case we can keep "add-ons").

chrome/content/zotero/standalone/standalone.js

@@ -625,6 +625,22 @@ const ZoteroStandalone = new function() {
 	this.updateAddonsPane = function (doc) {
 		//var rootWindow = doc.ownerGlobal.windowRoot.ownerGlobal;
 		
+		// Update message when no plugins installed
+		setTimeout(() => {
+			var emptyListMessage = doc.getElementById('empty-list-message');
+			emptyListMessage.innerHTML = Zotero.Utilities.htmlSpecialChars(
+					Zotero.getString("addons.emptyListMessage")
+				).replace(
+					/\[([^\]]+)]/,
+					`<a href="${ZOTERO_CONFIG.PLUGINS_URL}">$1</a>`
+				);
+			emptyListMessage.addEventListener('click', (event) => {
+				Zotero.launchURL(ZOTERO_CONFIG.PLUGINS_URL);
+				event.preventDefault();
+				event.stopPropagation();
+			});
+		});
+		
 		// Make our own removal prompt instead of using BrowserAddonUI.promptRemoveExtension() from
 		// browser-addons.js
 		doc.ownerGlobal.promptRemoveExtension = function (addon) {

chrome/locale/en-US/zotero/zotero.properties

@@ -1407,5 +1407,6 @@ tabs.duplicate = Duplicate Tab
 tabs.undoClose = Reopen Closed Tab;Reopen Closed Tabs
 tabs.closeOther = Close Other Tabs
 
+addons.emptyListMessage = Download plugins from the [Zotero plugins directory].
 addons.remove.title = Remove %S?
 addons.remove.text = Are you sure you want to remove %1$S from %2$S?

resource/config.js

@@ -30,6 +30,7 @@ var ZOTERO_CONFIG = {
 	LICENSING_URL: 'https://www.zotero.org/support/licensing',
 	GET_INVOLVED_URL: 'https://www.zotero.org/getinvolved',
 	DICTIONARIES_URL: 'https://download.zotero.org/dictionaries/',
+	PLUGINS_URL: 'https://www.zotero.org/support/plugins',
 };
 
 if (typeof process === 'object' && process + '' === '[object process]'){