But you have the dark mode in Settings + the Dark Reader addon.

I did absolutely nothing to my settings. One day, the interface was notly light colors. The next day, dark colors. That's not the only thing I dislike, however. I find it harder to search for a site, harder to select standard vs. private browsing, and who decided to put the menu bar or whatever you call it at the bottom of the screen instead of the top? I want to go back to the far superior version I had two months ago. The current one is terrible, at least for me with my visual impairment.

Hi,

Simply you can't,unless you are willing to uninstall it and install V68.11 from APKmirror.
In this way unfortunately you will loose your setting and bookmarks unless you backup 

your bookmarks before uninstalling firefox. After that do not update it with Google play and turn off automatic update at least for firefox.

This is the only way which  developers leave to us (which i did). 

This new version (from V79) is the main trouble and i ditched it completely.