How to get Mouse buttons 4 / 5 (Browser back / Browser forward) working in Firefox?

I can recommend the SensibleSideButtons project. It’s easy to install, and it will fix those mouse buttons in many other programs, too.

I had the same issue.

System Information

• macOS 10.15.5
• Firefox 83.0
• Logitech MX Master 2S Wireless Mouse

Steps to Reproduce

• Open Firefox.
• Navigate to page.
• Click back, click forward.

Resolution

• Install Logitech Options for macOS.
• Open application.
• Allow application to control your computer: System Preferences > Security & Privacy > Accessibility > Check Logi Options and Logi Options Daemon