Hmm, never seen that, and I'm using Firefox on a computer now.
There are text colour options in Preferences under General, Fonts and Colours (probably "Colors" with your language settings
). You can also find the Preferences as "Settings" via the hamburger menu on the right of your screenshot (where the Settings/Preferences are in the app menus is platform-dependent, as different operating systems have different norms, but that menu is common). There are also other colour options in "Add-ons and Themes" from the hamburger menu. If you have been playing with themes it's possible that this problem is a side-effect of an imperfect theme, so if you have customised the theme try changing the theme and seeing whether it solves the problem.
(I'm actually using the built-in "dark" theme customised using a new Mozilla add-on called "Firefox Color". This lets me have the reliability of the built-in theme but replacing the somewhat "meh" dark greys with deep blues and blue-greys, which suits me).