diff --git a/docs/configuration.rst b/docs/configuration.rst
index 9aed9675..107a8fa6 100644
--- a/docs/configuration.rst
+++ b/docs/configuration.rst
@@ -289,6 +289,19 @@ Description Source to read additional cookies from.
 =========== =====
 
 
+extractor.*.user-agent
+----------------------
+=========== =====
+Type        ``string``
+Default     ``"Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0"``
+Description User-Agent header value to be used for HTTP requests.
+
+            Note that this option has no effect on `pixiv` and
+            `readcomiconline` extractors, as these need specific values to
+            function correctly.
+=========== =====
+
+
 Extractor-specific Options
 ==========================
 
diff --git a/gallery_dl/extractor/3dbooru.py b/gallery_dl/extractor/3dbooru.py
index 3d084b0d..74c55346 100644
--- a/gallery_dl/extractor/3dbooru.py
+++ b/gallery_dl/extractor/3dbooru.py
@@ -17,7 +17,6 @@ class ThreedeebooruExtractor(booru.JSONBooruExtractor):
     api_url = "http://behoimi.org/post/index.json"
     headers = {
         "Referer": "http://behoimi.org/post/show/",
-        "User-Agent": "Mozilla/5.0",
         "Accept-Encoding": "identity",
     }
 
diff --git a/gallery_dl/extractor/chan.py b/gallery_dl/extractor/chan.py
index 4380d034..65e942d8 100644
--- a/gallery_dl/extractor/chan.py
+++ b/gallery_dl/extractor/chan.py
@@ -75,7 +75,6 @@ class FoolfuukaThreadExtractor(SharedConfigExtractor):
     def __init__(self, match):
         SharedConfigExtractor.__init__(self)
         self.board, self.thread = match.groups()
-        self.session.headers["User-Agent"] = "Mozilla 5.0"
         if self.referer:
             self.session.headers["Referer"] = self.root
 
diff --git a/gallery_dl/extractor/common.py b/gallery_dl/extractor/common.py
index d1cda0cb..8d00be97 100644
--- a/gallery_dl/extractor/common.py
+++ b/gallery_dl/extractor/common.py
@@ -34,6 +34,7 @@ class Extractor():
         self.session = requests.Session()
         self.log = logging.getLogger(self.category)
         self._set_cookies(self.config("cookies"))
+        self._set_headers()
 
     def __iter__(self):
         return self.items()
@@ -96,6 +97,13 @@ class Extractor():
 
         return username, password
 
+    def _set_headers(self):
+        """Set additional headers for the 'session' object"""
+        self.session.headers["Accept-Language"] = "en-US,en;q=0.5"
+        self.session.headers["User-Agent"] = self.config(
+            "user-agent", ("Mozilla/5.0 (X11; Linux x86_64; rv:54.0) "
+                           "Gecko/20100101 Firefox/54.0"))
+
     def _set_cookies(self, cookies):
         """Populate the cookiejar with 'cookies'"""
         if cookies:
diff --git a/gallery_dl/extractor/exhentai.py b/gallery_dl/extractor/exhentai.py
index a03c6945..9c79385c 100644
--- a/gallery_dl/extractor/exhentai.py
+++ b/gallery_dl/extractor/exhentai.py
@@ -50,11 +50,7 @@ class ExhentaiGalleryExtractor(Extractor):
         self.wait_max = self.config("wait-max", 6)
         if self.wait_max < self.wait_min:
             self.wait_max = self.wait_min
-        self.session.headers.update({
-            "User-Agent": "Mozilla/5.0",
-            "Accept-Language": "en-US,en;q=0.5",
-            "Referer": self.root + "/",
-        })
+        self.session.headers["Referer"] = self.root + "/"
 
     def items(self):
         self.login()
diff --git a/gallery_dl/extractor/readcomiconline.py b/gallery_dl/extractor/readcomiconline.py
index 0b44ddf2..d1b987f9 100644
--- a/gallery_dl/extractor/readcomiconline.py
+++ b/gallery_dl/extractor/readcomiconline.py
@@ -20,6 +20,10 @@ class ReadcomiconlineExtractor(kissmanga.KissmangaExtractor):
     filename_fmt = "{comic}_{issue:>03}_{page:>03}.{extension}"
     root = "http://readcomiconline.to"
 
+    def __init__(self, match):
+        kissmanga.KissmangaExtractor.__init__(self, match)
+        self.session.headers["User-Agent"] = "Wget/1.19.2 (linux-gnu)"
+
 
 class ReadcomiconlineComicExtractor(ReadcomiconlineExtractor,
                                     kissmanga.KissmangaMangaExtractor):
diff --git a/gallery_dl/extractor/sankaku.py b/gallery_dl/extractor/sankaku.py
index ff67b595..17f079b2 100644
--- a/gallery_dl/extractor/sankaku.py
+++ b/gallery_dl/extractor/sankaku.py
@@ -41,9 +41,6 @@ class SankakuTagExtractor(Extractor):
         self.wait_max = self.config("wait-max", 4)
         if self.wait_max < self.wait_min:
             self.wait_max = self.wait_min
-        self.session.headers["User-Agent"] = (
-            "Mozilla/5.0 Gecko/20100101 Firefox/40.0"
-        )
 
     def skip(self, num):
         pages = min(num // 20, 49)
diff --git a/gallery_dl/extractor/senmanga.py b/gallery_dl/extractor/senmanga.py
index d895824b..a4876f73 100644
--- a/gallery_dl/extractor/senmanga.py
+++ b/gallery_dl/extractor/senmanga.py
@@ -38,7 +38,6 @@ class SenmangaChapterExtractor(Extractor):
         self.chapter_url = "{}/{}/".format(self.root, part)
         self.img_url = "{}/viewer/{}/".format(self.root, part)
         self.session.headers["Referer"] = self.chapter_url
-        self.session.headers["User-Agent"] = "Mozilla 5.0"
 
     def items(self):
         data = self.get_job_metadata()
diff --git a/gallery_dl/extractor/twitter.py b/gallery_dl/extractor/twitter.py
index ff02070d..0d13077a 100644
--- a/gallery_dl/extractor/twitter.py
+++ b/gallery_dl/extractor/twitter.py
@@ -37,10 +37,6 @@ class TwitterTweetExtractor(Extractor):
         self.path, self.user, self.tid = match.groups()
 
     def items(self):
-        self.session.headers["User-Agent"] = (
-            "Mozilla/5.0 (X11; Linux x86_64; rv:48.0) "
-            "Gecko/20100101 Firefox/48.0"
-        )
         page = self.request("https://twitter.com/" + self.path).text
         data = self.get_job_metadata()
         imgs = self.get_image_urls(page)