Type about:config into your address bar, press Enter. Accept the warning that pops up

Then, search for browser.newtab.url. You can either right click it and click "Modify" to put in a custom page, or Right click and click "Reset" to restore the default.

I checked, and it is already set as about:newtab, but opening a new tab still takes me to the homepage.

(Img 1 is about:config before any changes. Img 2 is a newly opened tab.)

Ok, go ahead and try to Reset this preference: browser.newtabpage.enabled

It's also already on the default 'true'

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

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That did it. Thanks. :)