1- most recent version, eg 126.96.36.199 [latest] for 64bit win7, work in firefox,
BUT in add-on manager you see "McAfee Webadvisor Legacy",
and clicking on 'more' yields the following info:
"Note: Starting in Firefox 57, which will be released in November 2017, only add-ons built with this new technology will work in Firefox. These are indicated by the "Compatible with Firefox 57+" label on addons.mozilla.org (AMO). Add-ons built with the old technology are labeled "Legacy" in the Add-ons Manager (about:addons) tab.
If an add-on does not have the "Compatible with 57+" label or has the "Legacy" label, the developer may be in the process of transitioning to the new technology. ..."
I'm currently using FF 55.0.2 (64bit)
Hopefully McAfee will be better informed of FF changes and have transitioned b4 november... until the 4.0.6137 or later versions, i couldn't get FF webadvisor to work in FF.
p.s. Peter, given a previous issue w/webadvisor, i wanted to copy catdaddy [who assisted then] on this post but unclear to me how one add's a cc to a post...perhaps you can forward
addendum----meant to paste this, which gives the full explaination of Mozzila-Found FF new system: