I do not think that this is possible inside webextension.
But you can do it yourself.
If you use Firefox <57 just copy url from URLbar and paste on about:preferences page as home page.
If you use 57:
1. Open about:debugging page and copy UUID of "Bookmarked Speed Dial"
2. Open about:preferences page and paste as home page
moz-extension://your-UUID/data/index.html