This isn't reversible. Firefox really forgets about the website when you instruct it to, so there's no way to get the history back.

If there were a way to get it back, it would be a potential privacy violation since people use this command to remove traces of a website.

Firefox will remember new data from that website if you revisit. Data that was previously stored is gone as already posted above. You should be careful with using "Forget About This Site" because everything will be removed with no exceptions.

Firefox will forget everything from the selected site and that includes passwords, cookies, saved form data, exceptions (images, cookies, pop-ups) as well as bookmarks and history and cache items.