Hi Yiown,
Sorry for the delay in responding. I had to do some testing to get this figured out myself. After you have built the Engine from source code, you will also want to build the UnrealFrontend program.
This will give you an UnrealFrontend.exe file in your Engine\Binaries\Win64 folder. Running that will open the Unreal Frontend outside of the Editor. The window will have three tabs: Device Manager, Session Frontend, and Project Launcher. Select the Project Launcher tab, and at the bottom of that screen will be a section titled Custom Launch Profiles. On the far right side of the window in that section is a plus sign (+). Click that to create a new profile.
You will be taken to a page with a number of options. Double-click the name at the top to set a name, if you want. Set the Project drop down to Any Project. Make sure there is a check mark showing for the “Do you wish to build?” option. Set the Cook, Package, and Deploy options to Do not cook, Do not package, and Do not deploy respectively. Once these are set, an additional set of options will appear in the Build section. Select the HTML5 option and leave the rest blank. Your settings should look like this.
Click the Back button in the top right and it will return you to the Project Launcher screen with your new profile in the Custom Launch Profiles section. The second icon at the far right of your new profile shows the tooltip “Launch this profile” if you hover your mouse over it. Click the icon and let the process run. This creates the HTML5 folder containing UE4Game.js. Once the process is complete, open your project in the Editor and follow your normal process to launch the project on Firefox Nightly. This time it should launch successfully. Please let me know if you run into any additional problems.
The process I mentioned above is necessary when trying to launch a content-only Blueprint project for HTML5. If you have a code-based project, the process is slightly different. We are still working on streamlining the process of launching and packaging for HTML5. You will see some improvements to this process in the next few versions of the Engine.