Profiling in Firefox/Firebug, what am I looking at?

[Rob]

I am trying to track down the cause of some performance problems using
firebug, but I am not completely sure what that I am interpreting it
correctly. First of all, under the net tab, the total at the bottom
is the time from the submission of the request to the final
downloading of the last byte, correct? Or is it the time until any
onload javascript has finished executing? Furthermore, it is true
that execution of javascript in the page blocks downloading, isn't
it? Also, the progress bar in the status bar in Firefox nearly fills
up then empties and then fill up completely and stays that way for two
or three seconds and then finally goes away. Is the first part of
that the downloading from the server followed by the execution of the
onload javascript in those last two or three seconds?

Thanks for your help,
Rob