If the link is malicious, it can attempt to download some form of malware, redirect you to pages or try to pop up as many ads as it can do. Do you know how the link looks like it? (If you are posting it, censor it so it wouldn't be clickable). Because It seems like it's a malicious one.

Is this with all links, some, a few? Can you post some of those links here?

Check the Mailto application in Firefox > Preferences > Applications Make sure it isn't set to the Firefox application because Firefox can't handle the mailto protocol internally and you will get tabs opening endlessly.

See also:

• "Reset Download Actions": http://kb.mozillazine.org/File_types_and_download_actions
• https://support.mozilla.org/en-US/kb/Firefox+keeps+opening+many+tabs+or+windows