I don't know, I still use Firefox as my primary browser.
Seamonkey is pretty much the descendant of the OG Netscape Communicator codebase. So it's kind of on topic for retro Web stuff.
They were initially Netscape, a commercial company, so they had money from their customers.
After the browser code base was handed over from Netscape/AOL to the Mozilla Foundation in 2003, they got donations from AOL, IBM, Red Hat, etc. which kept them going for a few more years.
The Mozilla Foundation signed the deal with Google two years later, in 2005.
In short, they survived first on commercial revenues and then from donations, neither of which are substantial now.