Internet Explorer was certainly closed source, but it ran on many platforms.
It was popular on Mac Os (classic and X). It was also released for Solaris and HP-UX.
It was suffered on these platforms, because even IE for Mac didn't grant the 'compatibility' with 'web pages' designed for IE.
Internet Explorer on Mac was a completely different rendering engine (Tasman) to Windows (Trident). The only that was the same was the name.
(I swear at some point my brain will run out of space because it’s full of useless things like this.)