What do these numbers in brackets in the 'received' column mean in httpfox?

In httpfox I can see numbers in a pair of brackets in the received column.

It only happens to some but not all traffic.

What is the significance of it?

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Solution

This column as a whole represents the number of bytes received from the request.

Requests that are fulfilled from your cache, rather than being downloaded again, are shown in parenthesis. Thus the numbers in parenthesis are the number of bytes loaded from cache.

You can see that the HTTP Status code those requests received was 304 meaning the content hasn't changed and your browser should load from cache rather than downloading the file again.