Firefox on -current dumps core.

Hello.

I recently upgraded a Gateway MX6121 from 6.1 stable to -current, 
following the canonical procedure in /usr/src/UPDATING, and now whenever 
I try to start firefox, it dumps a core file (segmentation fault). 
Firefox was compiled from source under 6.1.

Should I have upgraded from 6.1 to -current, and /then/ start adding 
ports, or does that matter? I've done some preliminary googling, but not 
found anything that looks terribly recent or promising.

Any hints appreciated.