Thunderbird 24 would use GIO, while 17 used only GConf, so I suspect the latest reports would be something in GIO or the libgsettingsgconfbackend not escaping the '+' correctly.
Thunderbird 24 would use GIO, while 17 used only GConf, so I suspect the latest reports would be something in GIO or the libgsettingsgco nfbackend not escaping the '+' correctly.