It's very similar to the instructions for QTP and Firefox, except the extension is named a little differently:

1. Launch Firefox if it is not already open
2. Click Tools>>Add-ons. Alternatively, you can also type about:config in the url box and hit enter
3. Click Extensions on the left
4. In the list, find Unified Functional Testing Extension
5. Check the state of the add-on. If it is greyed out (disabled) click the Enable button to enable it.
6. Close the Add-ons manager in Firefox by clicking the 'X' in the upper-right corner of the window
7. Close Firefox browser
8. Make sure UFT is open. If not, launch it with the Web add-in selected. Double-check your record and run settings to make sure it is set to record and run on any open browser, or make sure Mozilla Firefox is selected in the specific browser drop-down if you need that option.
9. Once UFT is open, launch Firefox and test