That comes from the website not from Firefox browser.

That comes from the website not from Firefox browser.

So how I fix this problem?

You can check for issues with Enhanced Tracking Protection and Total Cookie Protection. Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content.

• click the shield icon for more detail and possibly disable the protection

You can check the Web Console for relevant-looking messages like about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

Ok, so what I have to do with Enhanced Tracking Protection and Total Cookie Protection? Is there a video guide or step by step guide?

So what sites is this that your seeing the captcha at?

Skelbiu.lt, kaina24.lt for example