Firefox returns (GIF Image, 1x1 pixels) when I click a link in Facebook

Hello Homanoxx.

You may be having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

Whenever you have a problem with Firefox, whatever it is, you should make sure it's not coming from one of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode and select Disable all add-ons. If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and theme. You can troubleshoot plugins the same way.

If you want support for one of your add-ons, you'll need to contact its author.

Also, be sure that your security software isn't interfering with your browser. Some antiviruses and firewalls are known to cause similar issues.

Some firewalls like Kaspersky can block content and send a 1x1 GIF image instead. Kaspersky": "Protection > Firewall > Filtration system: disable the "Banner Ad Blocker"

See Configuring Kaspersky Internet Security and Configure firewalls so that Firefox can access the Internet

Same problem but with Safari as well, so I'm guessing it's my (not Kapersky, but different) virus protection on my Mac.