The breakpoint is actually set at the line of the function definition, not at the line of the first instruction.
#7907 monitor() and debug() commands don't place breakpoints correctly
PR created. Honza, could you review please?
Also note that I work on a similar bug for the DevTools here: https://bugzilla.mozilla.org/show_bug.cgi?id=1164880
Merged into the master, closing.