Sometimes when I go to a site with social networking buttons (i.e. FaceBook, Twitter, Google+), a weird new tab opens up with a small button on the upper right corner. I suspect that it's an add-on problem; but because it occurs randomly, I cannot single out which add-on causes this weird behavior, nor can I provide an snapshot attachment immediately.
Note, The extensions Download Helper, DownThemAll, Fullscreen Homestarruner, TabMixPlus, and TACO were disabled at one point when this issue occurred.
If anyone reading this does not have this problem, please list the add-ons you use that are also on the Troubleshooting Information list so that I can rule them out.
The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.
To Reset Firefox do the following:
Go to Firefox > Help > Troubleshooting Information.
Click the "Reset Firefox" button.
Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
Firefox will open with all factory defaults applied.