diff --git a/http/config.go b/http/config.go
index 0232922e2f272098a011bcd404d8f0b73cc91ce1..a3a250152b22fb4e7203066cc6fc75942a55e0f6 100644
--- a/http/config.go
+++ b/http/config.go
@@ -3,6 +3,7 @@ package http
 import (
 	"net/http"
 	"net/url"
+	"strings"
 	"sync"
 
 	cors "github.com/rs/cors"
@@ -14,6 +15,19 @@ const (
 	ACACredentials = "Access-Control-Allow-Credentials"
 )
 
+// disallowedUserAgents specifies a denylist of user agents that are not
+// allowed to perform POST requests if they are not providing Origin
+// and/or Referer headers.  As mitigation for things like
+// https://bugzilla.mozilla.org/show_bug.cgi?id=429594.  Defaults to
+// Firefox-related things. The matching against the user-agent string
+// is made with strings.Contains().
+var disallowedUserAgents = []string{
+	"Firefox",
+	"Focus",
+	"Klar",
+	"FxiOS",
+}
+
 type ServerConfig struct {
 	// APIPath is the prefix of all request paths.
 	// Example: host:port/api/v0/add. Here the APIPath is /api/v0
@@ -30,6 +44,14 @@ type ServerConfig struct {
 	// websites to include resources from the API but not _read_ them.
 	AllowGet bool
 
+	// DisallowUserAgents specifies a blacklist of user agents that are not
+	// allowed to perform POST requests if they are not providing Origin
+	// and/or Referer headers.  As mitigation for things like
+	// https://bugzilla.mozilla.org/show_bug.cgi?id=429594.
+	// Defaults to ["Firefox"]. The matching against the user-agent
+	// string is made with strings.Contains().
+	DisallowUserAgents []string
+
 	// corsOpts is a set of options for CORS headers.
 	corsOpts *cors.Options
 
@@ -150,3 +172,31 @@ func allowReferer(r *http.Request, cfg *ServerConfig) bool {
 
 	return false
 }
+
+// allowUserAgent checks the request's user-agent against the list
+// of DisallowUserAgents for requests with no origin nor referer set.
+func allowUserAgent(r *http.Request, cfg *ServerConfig) bool {
+	// This check affects POST as we should never get POST requests from a
+	// browser without Origin or Referer, but we might:
+	// https://bugzilla.mozilla.org/show_bug.cgi?id=429594.
+	if r.Method != http.MethodPost {
+		return true
+	}
+
+	origin := r.Header.Get("Origin")
+	referer := r.Referer()
+
+	// If these are set, we leave up to CORS and CSRF checks.
+	if origin != "" || referer != "" {
+		return true
+	}
+
+	// If not, check that request is not from a blacklisted UA.
+	ua := r.Header.Get("User-agent")
+	for _, forbiddenUA := range disallowedUserAgents {
+		if strings.Contains(ua, forbiddenUA) {
+			return false
+		}
+	}
+	return true
+}
diff --git a/http/errors_test.go b/http/errors_test.go
index a320b7d7c16dfc393a282a0d081e4c501843871a..c55d42379b0d2feb08c31279135afac00f541d6c 100644
--- a/http/errors_test.go
+++ b/http/errors_test.go
@@ -170,3 +170,39 @@ func TestUnhandledMethod(t *testing.T) {
 	}
 	tc.test(t)
 }
+
+func TestDisallowedUserAgents(t *testing.T) {
+	tcs := []httpTestCase{
+		{
+			// Block Firefox
+			Method:   "POST",
+			AllowGet: false,
+			Code:     http.StatusForbidden,
+			ReqHeaders: map[string]string{
+				"User-Agent": "Mozilla/5.0 (X11; Linux x86_64; rv:10.0) Gecko/20100101 Firefox/10.0",
+			},
+		},
+		{
+			// Do not block on GETs
+			Method:   "GET",
+			AllowGet: true,
+			Code:     http.StatusOK,
+			ReqHeaders: map[string]string{
+				"User-Agent": "Mozilla/5.0 (X11; Linux x86_64; rv:10.0) Gecko/20100101 Firefox/10.0",
+			},
+		},
+		{
+			// Do not block Chrome
+			Method:   "POST",
+			AllowGet: false,
+			Code:     http.StatusOK,
+			ReqHeaders: map[string]string{
+				"User-Agent": "Mozilla/5.0 (Linux; U; Android 4.1.1; en-gb; Build/KLP) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Safari/534.30",
+			},
+		},
+	}
+
+	for _, tc := range tcs {
+		tc.test(t)
+	}
+}
diff --git a/http/handler.go b/http/handler.go
index b88e4e63619f6dcaabb549b05d3da80f2502b282..2152ea972367eede467fcee9789173b7c462ac19 100644
--- a/http/handler.go
+++ b/http/handler.go
@@ -121,7 +121,7 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if !allowOrigin(r, h.cfg) || !allowReferer(r, h.cfg) {
+	if !allowOrigin(r, h.cfg) || !allowReferer(r, h.cfg) || !allowUserAgent(r, h.cfg) {
 		http.Error(w, "403 - Forbidden", http.StatusForbidden)
 		log.Warnf("API blocked request to %s. (possible CSRF)", r.URL)
 		return