This is the standard download page:

https://get.adobe.com/flashplayer/

That page detects your browser and operating system and gives you a particular file that matches. For more choice, and to avoid extra software offers you might not want, you can get an installer from the following page:

https://www.adobe.com/products/flashplayer/distribution3.html

On that page, please ignore the stop sign section and scroll down to the first table. In the row for Firefox (Netscape-style plugin) you can get either the EXE or the MSI version, for most users, either one works equally well.

Does that work?

Oh, after downloading, Firefox should "animate" a blue arrow on the toolbar. Clicking that arrow shows a small panel with recent downloads. If you do not see that, try opening the full downloads list. Either:

• Ctrl+j
• (menu bar) Tools > Downloads

If you do not normally display the classic menu bar, you can activate it temporarily by pressing and releasing the Alt key.