Further to crakmunkey's post about using Webroot on a work computer, Webroot have responded to my request for more info on the issue:
"Our latest consumer release addressed the issues which our users were experiencing with Firefox. We would like customers running the business version of SecureAnywhere to contact Webroot Business Support with regards to any issues they are experiencing on their endpoints."
To reiterate, though, as of yesterday my consumer Webroot installation still caused the Firefox/Task Manager issue. A fresh install of Webroot yesterday killed keyboard input in MS Word and another text editor I use until I de-installed WR again.
As a long time user of WR with over year to go on my subscription it's very disappointing but I'm now using Malwarebytes.