If you are using Firefox on an iOS device, please be aware you will not have the option to use the Share Extension Clipper, per the Mozilla support site:
Add-ons for the desktop and Android version of Firefox are unavailable for Firefox for iOS.
Apple has created a proprietary iOS extension system which is incompatible with Firefox add-ons and extensions for Android, Mac, Windows and Linux. Also, iOS extensions written for other iOS apps won't work on Firefox for iOS (for example extensions written for Safari won't automatically work on Firefox for iOS).
We recommend using a different mobile browser on an iOS device to use the Share Extension Clipper.