I’ve installed the Push to Kindle extension in Firefox on Android 6.0, but there’s no button nor menu option that would allow me to use the extension. Can you please help? 
Hi there, sorry about that. We recently converted the addon to the new WebExtensions format that Mozilla is moving to. When we submitted it, it was marked as compatible with all platforms (including Android). Having looked into it now, it appears this is not the case. According to Mozilla docs, “Add-ons that work with desktop Firefox do not automatically work in Firefox for Android” - https://developer.mozilla.org/en-US/Add-ons/Firefox_for_Android
I’ve just removed Android from the compatibility section. Hopefully this gets reflected on the site and it’s no longer shown as compatible.
We do offer Push to Kindle as an Android app that’s compatible with Firefox and other browsers and apps (we are placed as an option in the list of ‘share’ destinations). I think it’s a better option for mobile as it’s designed to be used from small devices. It is, however, not free like this extension.