Install the apparmor profiles and extra profiles packages from the apt repository. They are sensible restrictions on common apps (web browsers) to prevent anything malicious from happening if they are ever hijacked. Make sure apparmor is enabled. This will do more to keep you secure than an antivirus. Maybe run your browser in a firejail for extra security.
If you insist on an AV, install ClamAV and have it scan weekly. It’s libre software and works well with Linux.
Thunderbird since forever. Before that, Seamonkey and the Mozilla suite.
There are some changes I didn’t like over the years like the tabbed interface for everything, but nothing else ever came along that worked as well and was multi-platform.