Anyway to fix this? Firefox displays HTML code...
 

... except only when viewing pages stored in exactly one folder on my computer. I honestly don't know why, the files are seen perfectly when opened in Firefox in the folder above the current one in the directly list, and in every other folder like it, just in a single folder, it simply displays the HTML code.

The HTML and CSS is perfectly valid, it's not a Mime Type problem because every other site and folder on my computer works perfectly fine when the pages are viewed in Firefox, just this one folder is apparently corrupted beyond any possible use because of it. Any suggestions, or should I just ask on the official Mozilla support forum?

I would ask at the support site of your webserver.

It's not a webserver though, it's a folder on my actual home computer, my actual site is fine, and every other folder on my computer is fine, just this one folder is somehow completely screwed.

set the file extension to open with w/e u want it too ;)

And are you not looking at that local directory using the webserver that is installed on your local PC?

If you're using Windows, try this...

In Explorer go to View -> Folder Options -> View. Uncheck "Hide file extensions for known types".

Look at the file again and make sure it doesn't have a double extension. If it is filename.html.txt than Firefox can show the source and not render it.

I'm not sure what's meant here, but usually, if you view a PHP file on your computer, it just shows the HTML content. However, for this sole directory, it just shows the whole thing in plain text, yet for all other folders it works fine.

Tried that, both the files in that folder and every other folder on my computer that were used had .php as the only file extension.

A PHP file is simply a text file. Firefox doesn't know that a PHP file can be processed and would treat it as any other unknown text file. If you want Firefox to treat a local file as HTML than name it as HTML. If you want to process PHP than install a webserver and PHP on your computer and process it as PHP through localhost and send the HTML output with the proper headers to Firefox.

Install XAMPP, WAMP, MAMP, IIS + PHP, whatever if you want to view PHP files on your local machine.

did you see my reply?

how about this, what OS is the computer running that your trying to view the file on?

Loco.M, it opens all other files with the same extension fine. But to answer your question, the computer in question is running Windows XP (yes, still, but I don't like Vista, and I'm relatively cheap with upgrading computers).