i've installed FF3.
i'm using r20467
OSX can switch between FF and Lazarus with not problems, by clicking on the icon docks as weel as pressing Cmd-Tab.
i've launched the Lazarus in the following ways:
1) (from lazarus dir) open lazarus.app
2) (from lazarus dir) lazarus.app/Contents/MacOS/lazarus
if you're using /Application/Lazarus bundle, with latest svn changes it might cause problems.