The browser.newtab.url pref is no longer supported in Firefox (41+), so it is no longer possible to set the new tab page via this pref.

You can look at this extension to set the new tab page:

• New Tab Override: https://addons.mozilla.org/firefox/addon/new-tab-override/

You can set the new tab URL via the Options/Preferences of the extension on the about:addons page.

You can look at this extension if you use the home page as the new tab page.

• New Tab Homepage: https://addons.mozilla.org/firefox/addon/new-tab-homepage/

Classic Theme Restorer includes this feature as well. You find "New tab page url" in the Tabs (2) section.

You can use pages like these:

• the default new tab page with the tiles is about:newtab
• the default home page is about:home
• for a blank page use about:blank
• for Super Start use about:superstart
• use the URL of a web page

See the about:about page for available about pages.

This works as long as I reset the addon and preferences every time I use Firefox. In other words, this is not a workable solution.

IVR4 said

This works as long as I reset the addon and preferences every time I use Firefox. In other words, this is not a workable solution.

When you say you need to reset the add-ons, do you mean Firefox always acts as if you are just installing it for the first time? That indicates that something is wrong with Firefox's extension registry files. Either Firefox is not able to write to them in the first place, or some external process is removing/replacing them between Firefox sessions. Same issue with your selected new tab page in the preferences file that should be saved between sessions.

Could you check on whether anything in this article seems applicable: How to fix preferences that won't save.