Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

While still in Safe Mode,

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

[v56+] Type about:preferences#general<enter> in the address bar. Under fonts and colors, press the Advanced button. Scroll to the bottom, turn on Allow Web Sites To Choose Their Own.

Under fonts and colors, press the Colors button and check the settings.

You can also look at; https://addons.mozilla.org/en-US/firefox/addon/nosquint-plus/ NoSquint Plus by Baris Derin

Manage site-specific zoom levels and color settings

Every time I open FF it's all in black. Can't even see any dialog.

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.

You can possibly delete prefs.js in the Firefox profile folder to reset all the prefs or you can check the content of this file and remove browser.display lines that have a color code as its value.

• user_pref("browser.display.background_color", "#000000");
• user_pref("browser.display.foreground_color", "#000000");

Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

"AppData" (XP: Application Data; Mac: ~/Library; Linux: ~/.mozilla) is a hidden folder.