The problem is that browser history is in no way related to the ability for you to browse offline content. In order to view the website offline, the entire website must have been stored locally, not just the URL.
In order to do what you propose, one of two things must be true:
1.) The page has been stored in your browser cache, which has not been purged.
2.) You must have saved the webpage to your hard drive while online.
To see what's in your Firefox browser cache, type about:cache into the address bar. Memory cache is lost when you close the browser, so the webpage needs to be saved in Disk Cache Device. Make sure that Firefox is not set to clear your cache on exit, and that you don't have any other programs that perform that type of cleanup. You can adjust the size of the Firefox browser cache by typing about:config into the address bar, then double-clicking browser.cache.disk.capacity.