mk_add_options doesn't work as people would expect it to intuitively; this is an issue most recently raised in bug 1603860. We currently don't have any safeguards in place to protect users from breaking themselves like this.
Unfortunately, the nature of the feature and its implementation means that
mk_add_options will behave as users intuitively expect it to sometimes but break in odd ways other times.
Unfortunately our public documentation suggests using
mk_add_options in places: https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Configuring_Build_Options
Addressing this bug fully can include adding additional errors/warning messages when someone attempts to set a variable with
mk_add_options that we know is broken as well as deprecating and removing
This is related to the unresolved bug 1413294 and bug 1089911.