Every time I try to change my Start Page it goes back to the default, how can I change it to one of my own liking?

I am having problems putting my own start page into Firefox 58.0.2. 64bit, it keeps going back to the default

I’ve moved this to the https://community.ubuntu.com/c/support-help-requests category because this site isn’t designed to take tech support questions. Sorry. I’d recommend using one of the official support channels listed on the https://community.ubuntu.com/t/finding-help/712 page.