With Space Next you can press the spacebar key at the end of a page to load the next page, just like you can do in Opera. This add-on tries to replicate Opera’s ability to automatically load the next page, hopefully, more successfully results-wise.
Here is a little more about the add-on from the developers:
In Firefox, pressing the “Space” key in a webpage usually makes it scroll down one page (page down) until the end of the page is reached. Opera browser has the same behavior, except that at the end of the page, the next page is automatically loaded (with more or less success), thanks to the “Fast Forward” function.
Want to try Space Next? You can pick up this add-on from the Firefox Add-ons website.