There has been a long standing conflict between Fast Dial Version 1.x and the Firefox Extension Web Developer

This was finally resolved in Fast Dial Version 2.x.
At the present moment Fast Dial Version 2.x can be downloaded here

However if you prefer for some reason to use Fast Dial Version 1.x and want a work-around to this problem, right click on FD, and go to Preferences/General and de-select Thumbnails in new Tabs.(see cursor position in picture)

This will cause Fast Dial Ver 1.x not to overwrite the Web Developer Information which displays in a new Tab.

You can choose to either leave this setting off, and use the Fast Dial Toolbar button to access FD, or
If for some reason you find it preferable you can change the Preference Setting back after using Web Developer.