Yes, many. Every WebKitGTK based browser [1] will use JSC, so that includes the "GNOME Web" web browser, Midori, and a host of smaller niche browsers.
Re:The endless browser wars
by MaxBarraclough on Friday February 05, 2021 @11:19AM
(#26037924)
Neat. Konqueror uses a different engine, right?
It can. Historically Konqueror used KHTML, however, the default has been changed to QTWebEngine, which is based on Blink i.e. Chromium.
It also supports (supported?) rendering using WebKit, or, you can manually change back to KHTML. However, all recent versions of Konqueror default to WebEngine. https://wiki.debian.org/Konqueror
Is it used in any fully Free and Open Source browser?