[v56+] Type about:preferences#general<enter> in the address box. Under Language and Appearance find Fonts and Colors, press the Advanced button. Scroll to the bottom, turn on Allow Web Sites To Choose Their Own.

Under Language and Appearance find 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

[v56+] Type about:preferences#general<enter> in the address box. Under Language and Appearance find Fonts and Colors, press the Advanced button. Scroll to the bottom, turn on Allow Web Sites To Choose Their Own. Under Language and Appearance find 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

Thanks but it didn't solve the problem, what happens is when you put your cursor ontop a tab, whichever tab, there will be a pop-up message saying the title of the tab, when I do it it happens what is in the picture, I cannot see what it says, I don't know if it can be solved with CSS

Do you mean the small window under "Get Support" that repeats "Get Support" ?

I think what you were on is the Bookmarks Toolbar. Not part of the web page.

I believe it is the pop-up you see when you hover over a tab. I have white text against a black background (since I don't remember when). I don't know how Windows themes affect what you see. You could try the following. Change layout.css.prefers-color-scheme.content-override in 'about:config' to: Dark (0), light (1), system (2) or browser (3) https://support.mozilla.org/en-US/kb/about-config-editor-firefox