Vugen also use the firefox but in silent mode. running TruClient with FireFox, and TruClient using vugen (when firefox is closed) may behave differently, mainly due to some gaps. Part of them due to bugs, and part of them due to limitation.
In order to ensure that BPM will run successfully your script, always run it in MDRV mode (None interactive mode)
I suggest to submit your question to LR forum with more details, and if necessary open a service request for additional help.