With these simple steps, you will be able to use Mozilla Firefox browser with FoxyProxy Standard Browser extension without a worry!
1. Open up Firefox and download FoxyProxy standard extension for Firefox.
2. Add extension to your Firefox browser. Press add.
3. Press on your extensions icon, open up your recently added extension, press options.
4. Once in foxyproxy menu, press proxies and press add.
5. Copy-paste credentials from your purchased proxy overview to corresponding fields in FoxyProxy and make changes to protocol type, if necessary. Save profile by pressing save.
6. After saving your profile, open up the extension again, and select your configured proxy.
7. Congratulations! You are now connected to proxy! Happy browsing!