If you don't have access to the SecureProxy page in Mozilla Addons Directory, you can install it from file. 

Step 1: download the extension file from our site.
Step 2: open Firefox menu > Addons (or press CTRL+Shift+A)
Step 3: click on the "Tools for all add-ons" button in the upper-right corner (the cogwheel icon) > Install add-on from file...
Step 4: browse and select the secureproxy.xpi file from the download folder