diff --git a/src/entrypoints/background/net-rules.ts b/src/entrypoints/background/net-rules.ts
index c0f9c9b..ad13d66 100644
--- a/src/entrypoints/background/net-rules.ts
+++ b/src/entrypoints/background/net-rules.ts
@@ -5,7 +5,8 @@ import Rule = chrome.declarativeNetRequest.Rule
 import RuleActionType = chrome.declarativeNetRequest.RuleActionType
 
 export async function applyUserAgentRule() {
-  // Set "MoodleMobile" User-Agent for requests to get_autologin_key
+  // Set "MoodleMobile" User-Agent for requests to get_autologin_key.
+  // This doesn't work in Firefox, so we need to set User-Agent in Axios.
   const ua_rule: Rule = {
     id: 1,
     action: {
diff --git a/src/shared/moodle-ws-api/axios.ts b/src/shared/moodle-ws-api/axios.ts
index e844edf..ec6a952 100644
--- a/src/shared/moodle-ws-api/axios.ts
+++ b/src/shared/moodle-ws-api/axios.ts
@@ -9,9 +9,9 @@ export const AxiosWS = axios.create({
   method: 'POST',
   headers: {
     'Content-Type': 'application/x-www-form-urlencoded',
-    // Browser doesn't allow to set User-Agent here.
-    // Extension should use declarativeNetRequest rules.
-    // "User-Agent": "MoodleMobile",
+    // Chrome prohibits to set User-Agent here. We should use declarativeNetRequest rules.
+    // But Firefox allows to set User-Agent here.
+    'User-Agent': 'MoodleMobile',
   },
 })