You could choose to open it with another browser such as Internet Explorer, and it should be easy enough to set the file so that clicking it opens it with your chosen application.

Rather like many other file types, you may have a choice of what you use when opening web pages. Taking another example a simple text file could be opened with say Notepad, or a complex word processor, or some other applications, including firefox.

Webpages may be opened with other applications, but if you are trying to view them the usual choice is to use a web browser. If you look at what has been saved you will probably see that a typical webpage is made up from a collection of files. Is there any particular reason you do not want to employ Firefox for opening offline web pages ?

PS
Long lines and 'pre' tag I will leave this as it is with the long line that you will need to scroll, I will post a question about that.
see now https://support.mozilla.com/en-US/questions/788460?new=1

Thanks for the reply!!!! I wanted to save and view a web page offline on my laptop that doesn't have internet access. I figured out what to do. I found an addon called SAVE AS PDF which does exactly what I wanted.

I think if the file is a 'PDF ' it should be easy to save and manipulate. We did not know from the original question what you were trying to do. see

• View PDF files using Firefox’s built-in viewer
• Use Adobe Reader to view PDF files in Firefox

OK so now that we know what you wanted to do this is the answer to your original question.

If you want to open a pdf file without using firefox either open adobe reader frist then open the file, or right click the file and choose open with adobe reader ( or any other chosen reader that handles such files) . When you take such action with the XP Operating System, you will get the choice to set such a reader as the default method of opening the files ; you can then open your offline pdf filed without using Firefox.