Every time I would click on certain files my professors would post, it would open up a new tab and ask if i wanted to open up the tab in firefox. I click yes, and it opens up a new tab and repeats. Eventually I clicked "do this with all files from now on" and now when i click on the link, even in a completely different browser, it opens up firefox to endless tabs and I have no way of stopping it other than closing the entire browser multiple times until it ends. How can I change this? It's extremely frustrating.

Unfortunately, your Firefox seems to be set to re-launch itself to view PPTX files. Since Firefox doesn't have the built-in ability to display PPTX files, it will keep sending the file to itself over and over and over, as you discovered.

Please see the following article to modify that setting and let PowerPoint (or whatever your presentation editor is) handle them instead:

Firefox repeatedly opens empty tabs or windows after you click on a link

But then, if it affects other apps as well, your Windows default file association for PPTX probably needs to be edited away from Firefox back to PowerPoint.

Note that you should never select Firefox as the application to handle a file if you get an ope with download dialog because Firefox wouldn't have shown this dialog if it could handle the file. An exception is PDF files send as a generic content type that allow to open the file in Firefox via the download dialog (Open in Firefox).