Array and object values may contain a lot more data as FirePHP traverses them (all array elements and object members) until the complete or maximum depth is reached. This is also true when logging exceptions and traces where all function and method arguments are also traversed. The data volume can quickly grow to megabytes which has fatal consequences for transmission of the debug data to the client and the client being able to render it.
The FirePHPPCore API gives you a way to set this maximum depth level bu setting either a "maxArrayDepth" or "maxObjectDepth" on the FirePHP object before sending it out to the logger. There's also a handy feature that, instead of limiting the depth, can filter out just the elements you're interested in. Code snippets for this and a Zend_Log_Writer_Firebug example are all included.