The Zen browser - a derivative of Firefox, also supports split tabs (and a lot of others like spaces, vertical tabs and glances). The project seems fairly active and up to date in the past few years.

Firefox has experimental split tab hidden inside about:config since 146.0 (December 9, 2025)

https://winaero.com/how-to-enable-split-view-in-firefox-146/

A decade ago there was a Firefox addon called Tile Tabs that could do any number of arbitrary splits. It died along with all the old-style XUL addons.

1. Look for a new browser instead of Firefox that also have a history of being trustworthy.

2. Look at Brave - see 1.