diff --git a/src/vector/index.js b/src/vector/index.js
index ec3cb4dc81..1d47491e4f 100644
--- a/src/vector/index.js
+++ b/src/vector/index.js
@@ -289,12 +289,23 @@ async function loadApp() {
                 // in case the Tinter.tint() in MatrixChat fires before the
                 // CSS has actually loaded (which in practice happens)...
 
-                // FIXME: we should probably block loading the app or even
-                // showing a spinner until the theme is loaded, to avoid
-                // flashes of unstyled content.
-                a.onload = () => {
+                // This if fixes Tinter.setTheme to not fire on Firefox
+                // in case it is the first time loading Riot.
+                // `InstallTrigger` is a Object which only exists on Firefox
+                // (it is used for their Plugins) and can be used as a
+                // feature check. 
+                // Firefox loads css always before js. This is why we dont use
+                // onload or it's EventListener as thoose will never trigger.
+                if (typeof InstallTrigger !== 'undefined') {
                     Tinter.setTheme(theme);
-                };
+                } else {
+                    // FIXME: we should probably block loading the app or even
+                    // showing a spinner until the theme is loaded, to avoid
+                    // flashes of unstyled content.
+                    a.onload = () => {
+                        Tinter.setTheme(theme);
+                    };
+                }
             } else {
                 // Firefox requires this to not be done via `setAttribute`
                 // or via HTML.