Hi,

try the following steps please:

Edit -> Settings -> Home -> Homepage and new windows: choose custom URL insert your url in the field. Tested in FF 94.0.2 (64bit) on a Manjaro Linux machine.

In Android I didn't found a way to set a custom startpage, but I want suggest a work around: You can add your custom startpage to the "top sites"

Add a site to your top sites Your top sites will be listed as icons on your homepage for easy access.

   Visit the page you want to pin.
   Tap the menu button. Settings icon in address bar in android
   Tap Add to top sites. 

When you open a new tab, the pinned website will now appear in your Top Sites.

@Mimicry, that only works with a basic url, not a string such as the one above. Go to StartPage, adjust the settings, copy the url, and try to paste it. You have to edit the search string. It won't do a straight copy/paste. I did end up pinning it, but it's annoyingly inefficient to have to click more than once to access web search.

Note that on desktop you can simply drag the tab with this page opened to the toolbar Home button to set this page as the homepage.

• https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

If you do not have the Home button on the toolbar (it isn't there by default) then drag the Home button out of the Customize palette to the toolbar.