Added instructions for manual Firefox install.

Author: manton

README.md

@@ -4,4 +4,12 @@ This simple extension adds a button to Firefox or Chrome to make it easier to bo
 
 It has been submitted to the Firefox and Chrome web extension stores. It can also be installed manually in those browsers.
 
+To install manually in Firefox:
+
+* Go to Firefox → Preferences
+* Click "Extensions & Themes"
+* Click the gear menu
+* Choose "Install Add-on From File..."
+* Download the release and select the .zip
+
 Safari support will follow later, as part of Micro.blog for macOS.