Many of the extensions on the Add-ons site are not compatible with Android due to its many differences from the desktop versions. If that is what the site is saying, I'm not sure there is a solution.

Someone posted a YouTube video that purports to show a way to install Greasemonkey on Android, but it has only 1 up vote and 11 down votes, which suggests that it doesn't work.

Sorry I don't have better news.

https://addons.mozilla.org/firefox/addon/greasemonkey/ is a desktop Firefox extension as the author has not made it compatible with Android Firefox also.

This is the section for Extensions that work on Android Firefox. https://addons.mozilla.org/android/

You can change the section at top where it says Other Applications.