Revert the default call to firefox

This reverts commit 0928a370f393fb5a416d0b8f8c78713d26c5ad87.

No, no, you guys are right in the comments for #2148. The call to
system is inside the else, but the tabbing made my eyes cross.
Sorry about that. Someday soon, @tabassassin will save us all from these
kinds of screw ups in mental parsing.