You cannot directly set a homepage. However, you can pin the page(which you need as your homepage) by unpinning a site from the top sites page and replacing it with your homepage url.

You can refer to this article: https://support.mozilla.org/en-US/kb/pin-sites-firefox-android-start-screen

Hi ace980

I’m replying to you because you posted a question about setting a homepage in the last few months.

We’re testing a homepage feature in Firefox Nightly and we’d love to get your feedback before we release it to the public!

Here are a few questions we have for you, but please include any other feedback:

1. Briefly describe what you expected the homepage in Firefox to be like.
2. How does the homepage in Firefox compare to your expectations?
3. Is anything missing from the ‘homepage in Firefox’ experience?
4. Was it easy or difficult to set your homepage in Firefox?
5. What is one thing you would change about the homepage in Firefox?

Ready to give it a try? Here’s how it works:

1. First, allow third-party app installation on your Android. Go to your Android Settings and enable 'unknown sources' (usually under "Security").
2. Download and install Firefox for Android Nightly at nightly.mozilla.org
3. Open Firefox for Android Nightly, then in the menu, tap Settings -> Customize -> Home -> Set a Homepage
4. Enter a homepage by either typing in a URL, or put a check mark next to ‘Use current tab’.
5. Finally tap ‘OK’ to set your homepage.

You’ll will see your homepage every time you restart Firefox for Android Nightly.

Cheers!

...Roland