Separated out disjoint resources by platform

src/platform/chrome/background.js

+/*	Enables the page action (i.e., the browser toolbar icon).
+	*/
+chrome.runtime.onInstalled.addListener(() => {
+	chrome.declarativeContent.onPageChanged.removeRules(undefined, () => {
+		chrome.declarativeContent.onPageChanged.addRules([{
+			conditions: [ new chrome.declarativeContent.PageStateMatcher({
+				pageUrl: { schemes: [ 'http', 'https' ] },
+			})
+			], actions: [ new chrome.declarativeContent.ShowPageAction() ]
+		}]);
+	});
+});
+
+/*	Toggle the toolbar icon based on whether stickies are, or are not, set to
+	be killed on the current tab.
+
+	This listener receives a message from the content script (contentScript.js);
+	the content script actually determines what the settings for the current
+	tab are.
+	*/
+chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
+	let icons = request.killingStickies ? {
+			"16":	"images/ASK_on_16.png",
+			"32":	"images/ASK_on_32.png",
+			"48":	"images/ASK_on_48.png",
+			"128":	"images/ASK_on_128.png"
+	} : {
+			"16":	"images/ASK_off_16.png",
+			"32":	"images/ASK_off_32.png",
+			"48":	"images/ASK_off_48.png",
+			"128":	"images/ASK_off_128.png"
+	};
+	chrome.pageAction.setIcon({
+		path: icons,
+		tabId: request.tabID || sender.tab.id
+	});
+});

src/platform/firefox/background.js

+/*	Enables the page action (i.e., the browser toolbar icon).
+	*/
+chrome.runtime.onInstalled.addListener(() => {
+	chrome.declarativeContent.onPageChanged.removeRules(undefined, () => {
+		chrome.declarativeContent.onPageChanged.addRules([{
+			conditions: [ new chrome.declarativeContent.PageStateMatcher({
+				pageUrl: { schemes: [ 'http', 'https' ] },
+			})
+			], actions: [ new chrome.declarativeContent.ShowPageAction() ]
+		}]);
+	});
+});
+
+/*	Toggle the toolbar icon based on whether stickies are, or are not, set to
+	be killed on the current tab.
+
+	This listener receives a message from the content script (contentScript.js);
+	the content script actually determines what the settings for the current
+	tab are.
+	*/
+chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
+	let icons = request.killingStickies ? {
+			"48": 	"images/ASK_on_48.png",
+			"96": 	"images/ASK_on_96.png",
+	} : {
+			"48": 	"images/ASK_off_48.png",
+			"96": 	"images/ASK_off_96.png",
+	};
+	chrome.pageAction.setIcon({
+		path: icons,
+		tabId: request.tabID || sender.tab.id
+	});
+});

src/platform/firefox/manifest.json

 		"default_popup":				"popup.html",
 		"default_title":				"AlwaysKillSticky - Click the icon to control sticky-killing on this site!",
 		"default_icon":					{
-			"16":							"images/ASK_on_16.png",
-			"32":							"images/ASK_on_32.png",
-			"48":							"images/ASK_on_48.png",
-			"128":							"images/ASK_on_128.png"
+			"48": 							"images/ASK_on_48.png",
+			"96": 							"images/ASK_on_96.png",
 		}
 	},
 	"icons":						{
-		"16": 							"images/ASK_on_16.png",
-		"32": 							"images/ASK_on_32.png",
 		"48": 							"images/ASK_on_48.png",
-		"128": 							"images/ASK_on_128.png"
+		"96": 							"images/ASK_on_96.png",
 	},
 	"options_ui":					{
 		"page":							"options.html",