I did a google search and that returns many hits,
https://www.google.co.uk/search?q=Uninstall+Secure+Browser.
The first of these was the official Avast Support article,
https://support.avast.com/en-gb/article/Uninstall-Secure-Browser was this the one you followed ?
However I don't believe you actually have Avast Secure Browser (ASB) installed, but you are seeing a misleading advert for Avast Secure Browser.
I have never installed ASB and have never come across this notification on my Avast Free Installations.
You mention this is opening a Tab in Firefox, so I'm just wondering if this is more related to the avast Security Browser extension, which is completely different. but confusingly named.
Clutching at straws a bit here:
In Firefox extensions/add-ons, do you have that extension installed (it may well be disabled, resulting in this notice), if so you should be able to uninstall it ?