If you really want to download an older version of Firefox, Mozilla keeps a full archive of old releases. However, you need to keep in mind that when you open an old version, it's immediately going to download the update and when you restart you will again have Firefox 75. To stop that, you would need to disable Firefox updates using Windows Group Policy or a policies.json file.

That said, running an older version of Firefox really isn't a good solution. It leaves you vulnerable to hundreds (if not thousands) or privacy and security holes that have since been patched.

A far better solution would be to figure out why the website you are having issues with is not working on the latest version of Firefox. Every website should work with Firefox and it's quick poor practice for a website to make a website incompatible with newer browsers.

If you share some more information about the website you are having trouble with and the features that are not working, we can probably help you to get them working in the current version of Firefox.

Functions in several websites no longer work since I upgraded past V71. I have contacted their support and was informed that Firefox V71 is the last release that works with the function I need.

What functions? It's hard to remember back to Firefox 72 now, but we might have some institutional memory of workarounds for some of the changes in that version if you can describe the problem in more detail.

(I think I'm repeating Wesley's comment in part, but for security reasons, it is important to keep up-to-date in your general browser.)