diff --git a/gallery_dl/extractor/common.py b/gallery_dl/extractor/common.py
index 16935bf3..5ada0305 100644
--- a/gallery_dl/extractor/common.py
+++ b/gallery_dl/extractor/common.py
@@ -42,8 +42,7 @@ class Extractor():
     ciphers = None
     tls12 = True
     browser = None
-    useragent = ("Mozilla/5.0 (Windows NT 10.0; Win64; x64; "
-                 "rv:128.0) Gecko/20100101 Firefox/128.0")
+    useragent = util.USERAGENT_FIREFOX
     request_interval = 0.0
     request_interval_min = 0.0
     request_interval_429 = 60.0
diff --git a/gallery_dl/util.py b/gallery_dl/util.py
index 3cbe5107..72ec98ee 100644
--- a/gallery_dl/util.py
+++ b/gallery_dl/util.py
@@ -647,13 +647,19 @@ class CustomNone():
     __repr__ = __str__
 
 
+# v128.0 release on 2024-07-09 has ordinal 739076
+# 735492 == 739076 - 128 * 28
+_ff_ver = (datetime.date.today().toordinal() - 735492) // 28
+
 NONE = CustomNone()
 EPOCH = datetime.datetime(1970, 1, 1)
 SECOND = datetime.timedelta(0, 1)
 WINDOWS = (os.name == "nt")
 SENTINEL = object()
-USERAGENT = "gallery-dl/" + version.__version__
 EXECUTABLE = getattr(sys, "frozen", False)
+USERAGENT = "gallery-dl/" + version.__version__
+USERAGENT_FIREFOX = ("Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:{}.0) "
+                     "Gecko/20100101 Firefox/{}.0").format(_ff_ver, _ff_ver)
 SPECIAL_EXTRACTORS = {"oauth", "recursive", "generic"}
 GLOBALS = {
     "contains" : contains,