This applies to Flexor versions before 2.1.59. To install the Salesforce Call Centre Adapter for Firefox, simply drag and drop Firefox_Salesforce_Add-on.xpi to an open Firefox window.
The Firefox add on can be found under Start - All Programs - Camrivox Flexor - Salesforce CTI sub menu: Firefox_Salesforce_Add-on.xpi or in the folder C:\Program Files\Camrivox\Flexor Manager\x.x.x.\Call Center Setup (where x.x.x. is the current version of Flexor).
If the adapter still does not appear in Firefox when logged into Salesforce please send the Flexor install log to support. The Flexor Install log file (InstallLog) is found in: C:\Program Files\Camrivox\Flexor Manager\x.x.x. (where x.x.x. is the current version of Flexor).