I hope I understand your situation correctly. It seems somewhat related to High contrast theme Firefox problem and to Need to modify gnome 3 theme for browser. And my answer below is essentially that which I provided to those questions!
Close (exit) all instances of Firefox.
Go to your profile folder. It is here: ~/.mozilla/firefox/randomstring.default. In there, look for a subfolder called chrome.
If it doesn't exist, create it.
To create the chrome folder you can use your file manager (or the command line with mkdir chrome).
If chrome does exist, look for a file called userContent.css. Otherwise, create an empty text file with this name in the chrome folder.
Now open userContent.css with a text editor and paste in or append this code:
INPUT, TEXTAREA {color: black !important; background: #aaaaaa !important; }
Save the file (as plain text) and close the text editor.
Restart Firefox.
You should now have black text on a light gray background. You can use whatever color combination you prefer.
Notes: chrome and userContent.css are case-sensitive and should be spelled correctly. The settings here will take precedence over those in the OS theme and will remain the same in Firefox irrespective of which gtk theme you switch to.