Setting up FVD speed dial in Firefox as my start page as a Custom URL in about:preferences#home; every time I launch Firefox, a blank page appears first and remains.
However, when I choose the Firefox Home (Default) as the start page (clearing all the Firefox Home Content checkboxes) the FVD Speed Dial eventually appears by itself.
It would be nice if the "moz-extension://744b7e8b-c07e-40dd-ae8c-296bfd167935/newtab.html" as the Custom URL option would work. I would then be able to use the Home button in the ribbon to return to my FVD Speed Dial.

After updating to FF Quantum v65.0, it's again not working as before.

Dear Gleb, Thank you!
I updated the add-on, and it's working well now.

Please disregard the above. It does not work again
....

I fixed my problem by:
1. uncheck the option "Show Speed Dial on startup" in the Speed Dial settings
2. restart Firefox
3. and then again open the Speed Dial options and again put the checkmark before "Show Speed Dial on startup".

4. The next time Firefox started, Speed Dial opened automatically, and the home button was also working properly.

I tried installing Firefox Developer Edition -- same story. The closest I could get to and FVD Startup is to choose Firefox Home (Default) as the Homepage and then clear all of the check boxes in the section Firefox Home Content in about:preferences#home. This will bring up FVD Speed Dial as the startup page albeit with a 2-4 second delay. The Home icon in the Menu Bar then becomes superfluous and can be removed. So to get back to the FVD Speed Dial, simply open a new tab :-)