Bookmarks are stored locally to your computer in your Firefox settings. Clicking a bookmark will take you back to the address you saved, loading the page currently available there (in other words, a bookmark isn't a copy of a page, it's just a URL). Bookmarks generally accumulate in chronological order and you can group them info folders. Using the Sync feature, you can share them with other copies of Firefox you have installed on desktop, laptop or Android devices that support Firefox. However, you cannot access them using other browsers.

Pocket is a web-based service with features to streamline web pages for easier reading. Addresses you add to Pocket are stored in the cloud by a company called Read It Later (with its own privacy policy). You can access pages saved to pocket using Firefox or any other browser, on a wide range of devices. Addresses are presented in a friendlier reading list format than traditional bookmarks.

Pocket offers a paid subscription service that can save a snapshot of the current page permanently (otherwise, like a bookmark, if a site removes a page, the address saved in Pocket is no longer useful).

Edit: This site has a bunch of help articles on Pocket. You can start here and then follow the links at the bottom for more info: Save web pages for later with Pocket for Firefox.

I should mention that you might want to use both. If there are addresses you need to visit routinely, you can bookmarks those, while if there are articles you want to read once, or videos you want to view once, and you might want to see them on a different device, it would make sense to send those to Pocket.