This fantastic add-on consists of strings or regular expressions to which matching pages stick in the form of bookmarks.
Here is more about this add-on from the developers:
Attach a regular expression to a folder within your Bookmarks Menu. If, in the course of browsing the Web, you encounter a page that matches one of your regular expressions, the a link to the page will be added to that bookmarks folder.
Want to learn more? You can pick up this add-on at the Firefox Add-ons website.