diff --git a/web/src/postprocess_content.ts b/web/src/postprocess_content.ts
index c2cafcdebf..b5c7edaf7f 100644
--- a/web/src/postprocess_content.ts
+++ b/web/src/postprocess_content.ts
@@ -108,7 +108,15 @@ export function postprocess_content(html: string): string {
         "div.message_inline_image > a > img",
     )) {
         inline_img.setAttribute("loading", "lazy");
-        if (inline_img.src.startsWith("/user_uploads/thumbnail/")) {
+        // We can't just check whether `inline_image.src` starts with
+        // `/user_uploads/thumbnail`, even though that's what the
+        // server writes in the markup, because Firefox will have
+        // already prepended the origin to the source of an image.
+        const image_url = new URL(inline_img.src, window.location.origin);
+        if (
+            image_url.origin === window.location.origin &&
+            image_url.pathname.startsWith("/user_uploads/thumbnail/")
+        ) {
             let thumbnail_name = thumbnail.preferred_format.name;
             if (inline_img.dataset.animated === "true") {
                 if (