diff --git a/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_oopif.html b/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_oopif.html
index 9e15da8971eb..4b162da3404f 100644
--- a/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_oopif.html
+++ b/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_oopif.html
@@ -50,7 +50,7 @@ async function setup_in_oopif() {
 
   const iframePromise = promiseOneEvent("OOPIF:SetupDone", null);
 
-  await FissionTestHelper.sendToOopif(testframe, setup.toSource() + "()");
+  await FissionTestHelper.sendToOopif(testframe, `(${setup})()`);
   await iframePromise;
 }
 
@@ -67,7 +67,7 @@ async function observe_styling_in_oopif(aFrameCount) {
   }
 
   const iframePromise = promiseOneEvent("OOPIF:StyleCount", null);
-  await FissionTestHelper.sendToOopif(testframe, observe_styling.toSource() + `(${aFrameCount})`);
+  await FissionTestHelper.sendToOopif(testframe, `(${observe_styling})(${aFrameCount})`);
 
   const styleCountData = await iframePromise;
   return styleCountData.data;
diff --git a/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_transformed_oopif.html b/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_transformed_oopif.html
index 862d3c77a01b..e6d914e2e625 100644
--- a/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_transformed_oopif.html
+++ b/gfx/layers/apz/test/mochitest/helper_fission_animation_styling_in_transformed_oopif.html
@@ -46,7 +46,7 @@ async function setup_in_oopif() {
 
   const iframePromise = promiseOneEvent("OOPIF:SetupDone", null);
 
-  await FissionTestHelper.sendToOopif(testframe, setup.toSource() + "()");
+  await FissionTestHelper.sendToOopif(testframe, `(${setup})()`);
   const rectData = await iframePromise;
   return rectData.data;
 }
@@ -64,7 +64,7 @@ async function observe_styling_in_oopif(aFrameCount) {
   }
 
   const iframePromise = promiseOneEvent("OOPIF:StyleCount", null);
-  await FissionTestHelper.sendToOopif(testframe, observe_styling.toSource() + `(${aFrameCount})`);
+  await FissionTestHelper.sendToOopif(testframe, `(${observe_styling})(${aFrameCount})`);
 
   const styleCountData = await iframePromise;
   return styleCountData.data;
diff --git a/gfx/layers/apz/test/mochitest/helper_fission_event_region_override.html b/gfx/layers/apz/test/mochitest/helper_fission_event_region_override.html
index d14d32d22d33..b0cc891e2912 100644
--- a/gfx/layers/apz/test/mochitest/helper_fission_event_region_override.html
+++ b/gfx/layers/apz/test/mochitest/helper_fission_event_region_override.html
@@ -41,7 +41,7 @@ let code_for_oopif_to_run = function() {
 async function* test() {
   let iframeElement = document.getElementById("testframe");
 
-  let iframeResponse = await FissionTestHelper.sendToOopif(iframeElement, code_for_oopif_to_run.toSource() + "()");
+  let iframeResponse = await FissionTestHelper.sendToOopif(iframeElement, `(${code_for_oopif_to_run})()`);
   dump("OOPIF response: " + JSON.stringify(iframeResponse) + "\n");
   ok(iframeResponse, "code_for_oopif_to_run successfully installed");
 
diff --git a/gfx/layers/apz/test/mochitest/helper_fission_scroll_oopif.html b/gfx/layers/apz/test/mochitest/helper_fission_scroll_oopif.html
index f1c32a5a8c45..40285f60edcc 100644
--- a/gfx/layers/apz/test/mochitest/helper_fission_scroll_oopif.html
+++ b/gfx/layers/apz/test/mochitest/helper_fission_scroll_oopif.html
@@ -59,7 +59,7 @@ async function* test() {
 
   let iframeElement = document.getElementById("testframe");
 
-  let iframeResponse = await FissionTestHelper.sendToOopif(iframeElement, code_for_oopif_to_run.toSource() + "()");
+  let iframeResponse = await FissionTestHelper.sendToOopif(iframeElement, `(${code_for_oopif_to_run})()`);
   dump("OOPIF response: " + JSON.stringify(iframeResponse) + "\n");
   ok(iframeResponse, "code_for_oopif_to_run successfully installed");
 
diff --git a/gfx/layers/apz/test/mochitest/helper_fission_transforms.html b/gfx/layers/apz/test/mochitest/helper_fission_transforms.html
index 7d84bba75ee8..c6d11798fb92 100644
--- a/gfx/layers/apz/test/mochitest/helper_fission_transforms.html
+++ b/gfx/layers/apz/test/mochitest/helper_fission_transforms.html
@@ -44,7 +44,7 @@ function failsafe() {
 async function* test() {
   let iframeElement = document.getElementById("testframe");
 
-  let iframeResponse = await FissionTestHelper.sendToOopif(iframeElement, code_for_oopif_to_run.toSource() + "()");
+  let iframeResponse = await FissionTestHelper.sendToOopif(iframeElement, `(${code_for_oopif_to_run})()`);
   dump("OOPIF response: " + JSON.stringify(iframeResponse) + "\n");
   ok(iframeResponse, "code_for_oopif_to_run successfully installed");