It is possible for the Firefox configuration to become malformed, if a user chooses non default options. The most common problem this can cause will be PDFs not being rendered in Firefox, but a download option being displayed.
Firefox can be returned to it's defaults, as originally set by the Tabernus Installer.
The easiest way to reset to factory defaults is to close Firefox, open a terminal (right click desktop, click open in terminal) and run the following command:
- rm -rf /home/tabernus/.mozilla
Restart Firefox and the problem should be solved.