It's called a "captcha" but you have given no hint at what url (/site) you are seeing this at. The captcha could be an image, you might not see third party images, it could be flash, you might be blocking an image with "Adblock Plus" or a hosts file. There may be other ways to depict an image and they might be blocked or missing a plug-in.

See:

• http://kb.mozillazine.org/Images_or_animations_do_not_load

There are several add-ons that could block a captcha. If you are trying to set up Firefox Sync, I would recommend you to start your browser in Diagnose Firefox issues using Troubleshoot Mode and set up the account.

After that, you can proceed as normal.

Also make sure that you do not block cookies.

• https://support.mozilla.com/kb/Cookies
• https://support.mozilla.com/kb/Enabling+and+disabling+cookies