Update website with Waterfox browser download information

[a-raccoon]

Thanks to another Issue posted on here, I learned about Waterfox for the first time -- a privacy-oriented fork of Firefox that still supports extensions like DownThemAll. You should tell people about it on your website downthemall.net

Since Firefox Quantum users can no longer use DownThemAll, and many are downgrading to older versions of Firefox at great risk to their security just to continue using DownThemAll, you might as well tell everyone to "upgrade" from Firefox to Waterfox instead.