By default Firefox fresh install assuming nothing change shouldn't do any redirects. How did you install Firefox? Was this wipe clean Firefox folder and reinstall or just reinstall over current Firefox?

Otherwise you will have to check with Firefox help link to be sure it's properly set.

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

Did a complete Firefox uninstall - restart of system - load new copy of Firefox from the link official FF website - all following some unresolved issue which FF troubleshooting suggested "refresh install". The custom URL which I utilized is one which I've been using for years without issue - I'm confident this isn't a faulty home page setup....I'm at a loss as what to do next.

Without someone being in person to see what uninstall and uninstall is doing this would be for a computer shop to see what is happening during the install of the Browser. A re-fresh usually wipes everything and there is no going back unless your system had image of the previous settings.