ok after quite a few tests, it seems that it works correctly with the run_firefox script every time.
It doesnt work if I open firefox normally (from launcher icon).
It does work if I open firefox from terminal
This suggests to that the .bashrc file isnt loaded properly. but then if I type echo $LD_PRELOAD in a terminal I get the path to the acroread_exec .so file so .bashrc file must be loaded.
So why wont it work properly if I open firefox from the launcher?