Using the Pocket Firefox Extension, you can access your reading list two different ways: the toolbar button and in the sidebar.
The Pocket Toolbar Button
Clicking the Pocket button in your Firefox Toolbar will reveal a popup list.
The button looks like this:
Note: if you cannot find the Pocket toolbar button, please make sure your Navigation toolbar is visible, and that the button has not been removed from the toolbar. Click here for more info.
The Pocket Sidebar
You can view your list as a sidebar instead - just go to View > Sidebar > Pocket. Alternatively, you can always use the sidebar instead of the popup list - just click the settings button in Pocket, and in the Appearance section, find 'Open reading list in', and choose 'in sidebar'.