--- a/mobile/android/components/extensions/test/mochitest/test_ext_tabs_executeScript_runAt.html
+++ b/mobile/android/components/extensions/test/mochitest/test_ext_tabs_executeScript_runAt.html
@@ -81,17 +81,17 @@ add_task(function* testExecuteScript() {
         // regardless of retries.
         browser.test.assertTrue(states[1] == "interactive" || states[1] == "complete",
                                 `document_end state is valid: ${states[1]}`);
         browser.test.assertTrue(states[2] == "complete",
                                 `document_idle state is valid: ${states[2]}`);
 
         // If we have the earliest valid states for each script, we're done.
         // Otherwise, try again.
-        success = (states[0] == "loading" || DEBUG &&
+        success = ((states[0] == "loading" || DEBUG) &&
                    states[1] == "interactive" &&
                    states[2] == "complete");
       }
 
       browser.test.assertTrue(success, "Got the earliest expected states at least once");
 
       browser.test.notifyPass("executeScript-runAt");
     } catch (e) {

--- a/toolkit/components/extensions/.eslintrc.js
+++ b/toolkit/components/extensions/.eslintrc.js
@@ -162,16 +162,19 @@ module.exports = { // eslint-disable-lin
     "no-invalid-regexp": "error",
 
     // No odd whitespace characters
     "no-irregular-whitespace": "error",
 
     // No single if block inside an else block
     "no-lonely-if": "warn",
 
+    // No mixing different operators without parens
+    "no-mixed-operators": ["error", {"groups": [["&&", "||"], ["==", "!=", "===", "!==", ">", ">=", "<", "<="], ["in", "instanceof"]]}],
+
     // No mixing spaces and tabs in indent
     "no-mixed-spaces-and-tabs": ["error", "smart-tabs"],
 
     // Disallow use of multiple spaces (sometimes used to align const values,
     // array or object items, etc.). It's hard to maintain and doesn't add that
     // much benefit.
     "no-multi-spaces": "warn",
 

--- a/toolkit/components/extensions/ExtensionCommon.jsm
+++ b/toolkit/components/extensions/ExtensionCommon.jsm
@@ -229,17 +229,17 @@ class BaseContext {
    * @returns {Error}
    */
   normalizeError(error) {
     if (error instanceof this.cloneScope.Error) {
       return error;
     }
     let message, fileName;
     if (instanceOf(error, "Object") || error instanceof ExtensionError ||
-        typeof error == "object" && this.principal.subsumes(Cu.getObjectPrincipal(error))) {
+        (typeof error == "object" && this.principal.subsumes(Cu.getObjectPrincipal(error)))) {
       message = error.message;
       fileName = error.fileName;
     } else {
       Cu.reportError(error);
     }
     message = message || "An unexpected error occurred";
     return new this.cloneScope.Error(message, fileName);
   }

--- a/toolkit/components/extensions/ext-c-runtime.js
+++ b/toolkit/components/extensions/ext-c-runtime.js
@@ -9,17 +9,17 @@ function runtimeApiFactory(context) {
 
       onMessage: context.messenger.onMessage("runtime.onMessage"),
 
       onConnectExternal: context.messenger.onConnectExternal("runtime.onConnectExternal"),
 
       onMessageExternal: context.messenger.onMessageExternal("runtime.onMessageExternal"),
 
       connect: function(extensionId, connectInfo) {
-        let name = connectInfo !== null && connectInfo.name || "";
+        let name = (connectInfo !== null && connectInfo.name) || "";
         extensionId = extensionId || extension.id;
         let recipient = {extensionId};
 
         return context.messenger.connect(context.messageManager, name, recipient);
       },
 
       sendMessage: function(...args) {
         let options; // eslint-disable-line no-unused-vars

--- a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_upload.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_upload.html
@@ -80,17 +80,17 @@ function background() {
     browser.test.log(`onBeforeRequest upload: ${details.url} ${JSON.stringify(details.requestBody)}`);
     browser.test.assertTrue(!!requestBody, `Intercepted upload ${details.url} #${details.requestId} ${upload} have a requestBody`);
     if (!requestBody) {
       return;
     }
     let byteLength = parseInt(upload, 10);
     if (byteLength) {
       browser.test.assertTrue(!!requestBody.raw, `Binary upload ${details.url} #${details.requestId} ${upload} have a raw attribute`);
-      browser.test.assertEq(byteLength, requestBody.raw && requestBody.raw.map(r => r.bytes && r.bytes.byteLength || 0).reduce((a, b) => a + b), `Binary upload size matches`);
+      browser.test.assertEq(byteLength, requestBody.raw && requestBody.raw.map(r => r.bytes ? r.bytes.byteLength : 0).reduce((a, b) => a + b), `Binary upload size matches`);
       return;
     }
     if ("raw" in requestBody) {
       browser.test.assertEq(upload, JSON.stringify(requestBody.raw).replace(/(\bfile: ")[^"]+/, "$1<file>"), `Upload ${details.url} #${details.requestId} matches raw data`);
     } else {
       browser.test.assertEq(upload, JSON.stringify(requestBody.formData), `Upload ${details.url} #${details.requestId} matches form data.`);
     }
   }

--- a/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_storage_sync.js
@@ -116,17 +116,17 @@ class KintoServer {
   installBatchPath() {
     const batchPath = "/v1/batch";
 
     function handlePost(request, response) {
       let bodyStr = CommonUtils.readBytesFromInputStream(request.bodyInputStream);
       let body = JSON.parse(bodyStr);
       let defaults = body.defaults;
       for (let req of body.requests) {
-        let headers = Object.assign({}, defaults && defaults.headers || {}, req.headers);
+        let headers = Object.assign({}, (defaults && defaults.headers) || {}, req.headers);
         // FIXME: assert auth is "Bearer ...token..."
         this.posts.push(Object.assign({}, req, {headers}));
       }
 
       response.setStatusLine(null, 200, "OK");
       response.setHeader("Content-Type", "application/json; charset=UTF-8");
       response.setHeader("Date", (new Date()).toUTCString());
 

--- a/toolkit/modules/addons/WebRequest.jsm
+++ b/toolkit/modules/addons/WebRequest.jsm
@@ -57,17 +57,17 @@ var RequestId = {
     let id = (this.count++).toString();
     if (channel) {
       getData(channel).requestId = id;
     }
     return id;
   },
 
   get(channel) {
-    return channel && getData(channel).requestId || this.create(channel);
+    return (channel && getData(channel).requestId) || this.create(channel);
   },
 };
 
 function runLater(job) {
   Services.tm.currentThread.dispatch(job, Ci.nsIEventTarget.DISPATCH_NORMAL);
 }
 
 function parseFilter(filter) {