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1. Press the Windows logo orb on the taskbar to bring up the Start Menu.
2. In the search box, type Fonts then press Enter.
3. In the font manager window, if you have the Helvetica font installed, delete it, along with all variants, like Helvetica Neue (N.B.: these fonts are only problematic on Windows). Restart Firefox afterwards.

There's no option to upload an image when you post the question (as far I could see). But thanks, here I see the button.

I can't just delete fonts, I'm a graphic designer. This problem started randomly in this days, no new topographies have been installed and Helvetica families have been in my system for years.

I'll try just to reinstall Firefox, gotta save all the bookmarks.

https://support.cdn.mozilla.net/media/uploads/images/2014-04-11-08-09-16-911af7.png

Your picture looks like an old punch card that computers used to use.

Clear the Cache Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

and

Remove Cookies Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

If there is still a problem, Start Firefox in Safe Mode While you are in safe mode;
Firefox Options > Advanced > General.
Look for and turn off Use Hardware Acceleration.

This could be a problem with the Helvetica Neue font, so if you have this font installed then try to uninstall it temporarily as a test.

• body { font-family: "HelveticaNeue","Helvetica Neue",Helvetica,Arial,sans-serif; }

Well I tryed reinstall Firefox without success.

YES! It looks like one of those hahahaha

I deleted the Helvetica's and succeed. After all, you were right, thanks Gingerbread Man and cor-el.

If I wanna keep my fonts, your last option FredMcD did it (Turn off Hardware acceleration) :)