In firefox, some websites will have the text edit area backgrounds colored in a similar way to native apps when using the dark theme. This makes it difficult to read the text, as the text color stays black.
This one for example: www.bustaname.com
I have had a similar thing happen with other GTK themes recently. With the Lhun-Magol theme it also makes some text boxes dark. I think this was caused by a recent change to how Firefox supports GTK theme, as this only occurred recently after a Firefox update.
experimented a bit - using gnome-tweak, setting a global dark theme has the same effect.
Both arc dark for gtk+firefox themes, it's common problem for all dark themes I have seen.

EDIT: Yes, running firefox with dark theme selected and light theme enviroment works awesomly good "env GTK2_RC_FILES=/usr/share/themes/Arc/gtk-2.0/gtkrc firefox"
Relevant bugzilla https://bugzilla.mozilla.org/show_bug.cgi?id=519763 This is firefox issue not the theme's issue.
In about:config, there is usseful browser.display.use_focus_colors option that will higlight active inputbox.
Then you can set browser.display.focus_background_color and browser.display.focus_text_color to black and white if you wish. This will only increease readibility, I appeal to you guys to notify Mozilla that this is extreamly frustrating and that they should fix the browser.display.use_system_colors;false cause this behavior is angrysome
And if you can't bare it at all, you can change the desktop entry of firefox and change exec entry to
Exec=bash -c 'GTK_THEME=Adwaita:light firefox %u'
It will use completly different schema. I didn't try it with desktop entry with adwaita + Arc dark theme in firefox, might completely solve the issue. :) Try it and give us feedback
EDIT: Yes, others run please firefox like this
~$ env GTK2_RC_FILES=/usr/share/themes/Arc/gtk-2.0/gtkrc firefox
It will still be dark but Gtk will be light, thus not cripling input boxes
You could also look here for a workaround http://knusper.net/knusperblog/2014/firefox-gtk-dark-theme-fix/
It's closed because the problem is is with Firefox, not the theme. You can't expect a theme to fix Firefox's issue.
I was walking around and was hoping I can fix this issue then I saw that 17 years old bug lol.
Most helpful comment
https://bugzilla.mozilla.org/show_bug.cgi?id=70315
Yeeeesh