You can see here:

• How to set the home page

Or try go to about:config search for browser.startup.homepage and change its value to:

about:home default

about:blank blank page

url one page that you want

• about:config

Open a new window or tab. In the address bar, type about:config. If a warning screen comes up, press the Be Careful button. This is where Firefox finds information it needs to run. At the top of the screen is a search bar. Enter browser.newtab.url and press enter. browser.newtab.url tells Firefox what to show when a new tab is opened. If you want, right click and select Modify. You can change the setting to;

about:home (Firefox default home page),
about:newtab (shows the sites most visited),
about:blank (a blank page),
or you can enter any web page you want.

The same instructions are used for the new window setting, listed as browser.startup.homepage.

I don't think the tricks from Firefox for desktop are applicable to the start page of Firefox for Android. Unless something has changed in the past few versions, it cannot be replaced with a different page.