What is the meaning of questionmark in firebug net tab?

Here is the print screen. I have marked the size column with red rectangular. alt text http://img85.imageshack.开发者_运维百科us/img85/1117/fbug.png Full image link

Added link for Expanded headers

Yes, the question mark is displayed in cases where Firebug couldn't figure out the size of the response (e.g. missing Content-Length header). This is considered rather as a bug and should be fixed in the combo: Firefox 3.6 + Firebug 1.5 Honza

See this bug report. Problem appears to be the lack of a Content-Length header for those file.