Why do you HAVE to? Why not wait for F-Droid to build the new version in the next build cycle?
You can’t upgrade the F-Droid one with the Divest one, Android security feature.
I can see them just fine:
https://f-droid.org/en/packages/org.mozilla.fennec_fdroid/
https://f-droid.org/en/packages/us.spotco.fennec_dos/
…they did not go anywhere,