Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

I'm using firefox in ubuntu. When I switch to Windows 10, those website come to normal. But it still continue in ubuntu. please help me.

Did you check the default text color and background color?

• Options/Preferences -> General: Fonts & Colors -> Colors

I did. but it does not work. I'm using ubuntu 17.04.

I've had this problem and it's converse. Many sites show up as a black box, and text I enter is black or very dark grey.

   Until I find out what is wrong, there is a kludge.  I can type my answer 'blind' and check and correct by highlighting the region where I typed -- my text shows up.  If I move the cursor to an error and click, everything disappears again, but if I remember the error, I can type a backspace and remove the wrong letter, type in the missing letter, whatever.
   Hardly what I'd call a "solution", but if you really HAVE to fill out some form, it can be done, with patience.
   The problem arose when I began experimenting with 'night mode,' 'dark theme,' 'Dark Background with Light Text,' and so on.  Worked beautifully in most places, but not everywhere.  I sometimes get a printout with black text on a white background, even though the word processing screen shows light on dark.  Perfect.  But then on some web pages, it will print as white letters on black, eating toner at 25 times the normal rate.  Worse is an all-black page.
   Very strangely, I just tried to print out a selected solution to a different problem from this very Forum, and in the print preview the first page looked just like the web page, but the second page was all black.  Pages 3 and 4 didn't show at all.
   I've undone, removed, reset everything I can remember, but here and there this kind of thing still pops up.
   Staring at a CRT with some amber letters on the screen for hours was bad enough.  Then I started using a Xerox PARC Alto.  Vertical orientation like a "page," black characters on a white "page."  A paperless office!  But the eye strain was much worse.  Where is 'choice'?  Glaring screens are universal.
   I don't think staring at a few fluorescent lights for hours is a good idea.  I'm still going to look for something less damaging to my eyes.  I've successfully set DuckDuckGo, LibreOffice, and Firefox individually to amber, yellow, and white on a dark grey background.  Playing with Windows 7 High Contrast theme, and a couple of add-ons were less successful. Black screen, but button labels of DARK green, DARK blue. Sometimes printing the web page in B&W would show the labels, and you could hold up the page to the screen and count the yellow outlines to get to the button you needed.
   Still undoing whatever I can remember or find.  There is no universal "reset to original" button though.  Be careful if you try it, and keep notes as you go.
   Good luck, and if you find an answer, please tell us.

GNUbee said

I can type my answer 'blind' . . . . .

Open a text program. Write your message. Then copy it to the browser.

Are you using code in userChrome.css and userContent.css to modify colors?