This is odd, so I'll try and explain as best as possible. All the links to the SM icons work in Safari. No matter what I do, Twitter, Facebook and Google+ will not open in Firefox unless I right-click on the icon and open in new window. They will not open by clicking or by right-click and opening in a new tab. Only a new window.
To see if it was the position (ANY top three icons) I removed the social media profiles from Twitter, Facebook and Google+, making the next three SM profiles/icons move up to the top row. THEY worked fine. So it isn't the placement and it isn't the user name. There is something wrong with the HTML code for Twitter, Facebook and Google+ that causes Firefox on a Macintosh not to open the webpages.
You can see for yourself (as of now the plugin is activated…however it may not be for long.)