Issue:
There’s an issue when using a dark theme in some Linux distros, Firefox text entry fields don’t show visible text. The only way to see what, if anything is entered is by using the mouse to highlight the whole area.
This gets annoying really fast, it’s particularly evident on sites such as banking and government services.
The problem is down to how Firefox interacts with certain GTK dark themes.
Fix:
With Firefox this issue is simple to fix.
Type about:config in the search bar.
Accept the ‘Proceed with caution’ warning.
Click ‘Show All‘ to list all of the available config options.
Scroll down the list to where you see widget.content.gtk-theme-override.
It likely has nothing in the value section.
Enter Adwaita as the value and restart Firefox.
You should now find that the previously invisible text is visible.
If the problem persists you might not have the Adwaita theme installed (this is rare, it’s basically a default theme at this point.) Find a light theme that is installed and enter that instead of Adwaita.