Click "File -> Open File...", and open the file "swfcatcher.xpi". Then you can see the Software Installation window. Click "Install Now" to install SWF Catcher for Firefox.
You may not see the above dialog box but a yellow Popup Blocker as the following illustration:
In that case, please click the Edit Options button which besides the Popup Blocker. And then click Allow button as follows:
Restart Firefox. You will find SWF Cathcer for Firefox was successfully installed.