Mozilla has disabled my previous ad and privacy blocker on Firefox Quantum 66.0.3 (64-bit) for Windows. So I found malwarebytes browser extension using the link you provided.
However, I could not download this beta version of the extension directly from Firefox Add-Ons webpage. I keep getting the error " Download failed. Please check your connection."
My wifi connection is working perfectly fine and is still connected to the internet so I don't know what the real issue is.
Is there another way of downloading the Malwarebytes browser extension from another download site?