For some reason, I’m using the same Firefox configuration since ages. Today, when Asana insisted that my browser was too old, I was a bit puzzled since I’m running the latest Fedora and I was reasonably sure Firefox was updated. It ended up being an old TLS setting.
To change it, open a new tab and type about:config in the address bar. In the new page, search (there’s a search box) for the key security.tls.version.min and make sure it’s set to 3.