Hm... I know that Firefox uses CSS, it's mostly used to change the appearance of websites, including colors and the like.
There's this page on Reddit which gives you a quick rundown of where the files are: https://old.reddit.com/r/FirefoxCSS.....efox_with_css.
You'll probably need to indicate that the text needs to be white on an inactive tab. A quick search gave me this page back: https://www.tutorialpedia.org/blog/.....ab-text-color/
If that page is correct you would probably need to add tab:not([selected]) .tab-label { color: white;} to one of the theme CSS files

user_pref("userChrome.theme.built_in_contrast", true);
user_pref("userChrome.theme.system_default", false);
user_pref("userChrome.theme.proton_color", true);
user_pref("userChrome.theme.proton_chrome", true); // Need proton_color
user_pref("userChrome.theme.fully_color", true); // Need proton_color
user_pref("userChrome.theme.fully_dark", true); // Need proton_color

user_pref("userChrome.theme.system_default"
if i turn it to false, it goes back to be blue with black letters, while the active tab is grey with white letters
If i turn it to true, i'm back to black writing on grey background

Just not grey with white letters for all non active tabs

Tried a lot of things now and it works everywhere besides for the actual toolbar and tabs

Hm... did you try tagging on an !important at the end? That forces the rule for that text color.

So that would make the rule:
tab:not([selected]) .tab-label { color: white !important;}

Took a while but managed to change the font color in the tabs
Now i'm just missing the font color in the menu bar

Found it
menubar > menu {
color: #FFFFFF !important;
}

I can see why people get a kick out of this
I just completely changed the hover options and other stuff just how i like them

Hehe, I have worked with CSS, and I know that feeling.

Until we're talking about vertical centering before CSS3....