Could be, I find portable versions of programs can be very slow depending on the drive they're running from.
What I do notice, however, is that the full menus are displayed for a split second straight after Firefox loads, and then the extension kicks in and hides them.
If you're experiencing slow start up time for Firefox, try starting from scratch again - uninstall it,. remove all traces, and then install it again.
I did, and it got me from 20 seconds to 5 seconds start time. Also, defragment your U3 drive, lots of people don't bother defragmenting their USB drives but it can be worthwhile.
