the current '3.0' version is still in the 'release candidate' stage... --many-- bugs are likely to be found... and just as quickly squashed...
if you are worried stick with version 2 till a few security update releases of FF3 come to the surface...
[EDIT]
the RC2 (windows/linux) and RC3 (mac) versions are considered final... only thing left is the bug smooshing...
tho i can honestly say ive been using FF3 since beta 2 and havent had any troubles with it (AVG is keeping an eye out but nothing has happened)