Firefox has integrated the Pocket button into the browser since version 38.0.5. You can take out the Pocket button by navigating to the customize button of Firefox.
But I just don’t like the new design. I cannot tell if a webpage was Pocked already or not when I browsed it the second time. And I had to open up a web page to view my Pocked list. So I decided to take the old Pocket button back (sorry, Mozilla). If you feel the same way, you can download the old Pocket add-on file for Firefox from here. Please leave a comment if you have difficult downloading this file and please identify your region.
After you download this file, you shall be able to install it by simply double clicking. If not, go to the Add-ons page in Firefox, click the gear (‘tools for all add-ons‘) icon on the top-right, select ‘Install Add-on From File…‘ and select the downloaded .xpi file.
Enjoy your retro Pocket button 🙂