There are a number of ways you can save to Pocket using the Firefox Extension.
The Pocket button appears in Firefox's Navigation Toolbar (a.k.a. Location Bar, URL Bar), and a single click will save the current page to Pocket. It will turn red, indicating that the page has been saved.
Later, when you're finished reading the page, click the red Pocket icon to send the page to the Archive.
The Right-Click Menu
If you right-click on the page background, you will see an item in the Context Menu to "Save to Pocket".
Via Keyboard Shortcut
If you prefer using keyboard shortcuts, you can press Alt/Option + W to save the current page. There are a number of other keyboard shortcuts as well - to see the entire list, open Pocket, click Settings, and select Keyboard Shortcuts. Here you will see a list of available shortcuts, and have the opportunity to edit them.
Saving all Open Tabs
Pocket can save all of your open tabs at once. To do this, simply right-click on a tab in the Tab Bar, and in the context menu, choose "Save All Tabs for Later".