diff --git a/browser/themes/shared/preferences/dialog.css b/browser/themes/shared/preferences/dialog.css
index 7373e8af5efa..ed63460aa5c4 100644
--- a/browser/themes/shared/preferences/dialog.css
+++ b/browser/themes/shared/preferences/dialog.css
@@ -2,12 +2,6 @@
    - License, v. 2.0. If a copy of the MPL was not distributed with this file,
    - You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-:root {
-  /* FIXME bug 1784891: This duplicates the font-size set in common-shared.css,
-   * as that stylesheet sets the font size only for HTML documents. */
-  font-size: 15px;
-}
-
 window,
 dialog {
   appearance: none;
diff --git a/toolkit/components/prompts/content/commonDialog.xhtml b/toolkit/components/prompts/content/commonDialog.xhtml
index f9e378adda38..eac3bf3fca77 100644
--- a/toolkit/components/prompts/content/commonDialog.xhtml
+++ b/toolkit/components/prompts/content/commonDialog.xhtml
@@ -16,6 +16,7 @@
         xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
         aria-describedby="infoBody"
         headerparent="dialogGrid"
+        class="system-font-size"
         onunload="commonDialogOnUnload();">
 <dialog id="commonDialog"
         buttonpack="end">
diff --git a/toolkit/themes/shared/in-content/common-shared.css b/toolkit/themes/shared/in-content/common-shared.css
index 16d9d19e08e4..459997ad3b0f 100644
--- a/toolkit/themes/shared/in-content/common-shared.css
+++ b/toolkit/themes/shared/in-content/common-shared.css
@@ -264,12 +264,7 @@
   color: var(--in-content-page-color);
 }
 
-/* FIXME bug 1784891: For historical reasons, we hardcode a font-size only for
- * HTML root nodes. XUL root nodes using this stylesheet get their font-size
- * from `font: message-box` above. Having the choice of markup language affect
- * the font-size like this seems arbitrary, so we should probably consolidate
- * this at some point. */
-html:root {
+:root:not(.system-font-size) {
   font-size: 15px;
 }