What is meaning of 'Blocking' in Firebug Net Panel?

Question

I\'m using Firebug 1.5.2 and while testing a site before production release i can see a huge amount of time consumed by \'Blocking\' parts of the requests.

What exa

Answer 1

It means "waiting for connection". As explained in the official documentation by Mozilla, "Blocking" is "Time spent in a queue waiting for a network connection." That can be due to Firefox hitting its internal parallel connections limit, as explained there and in answers here.

It can also mean "waiting because server is busy". One possible reason for "Blocking" times is missing in the official documentation linked above: it can happen when the server cannot provide a connection at the time because it is overloaded. In that case, the connection request goes into a queue on the server until it can be processed once a worker process becomes free [source].

In a technical sense, such a connection is not yet established because the request is awaiting accept() from the server [source]. And maybe that is why it is subsumed under "Blocking" by Firefox, as it could also be considered "Time spent in a queue waiting for a network connection".

(This behaviour is not fully consistent as of Firefox 51 though: for the first URL called up in a new tab, the time before the server accepts the connection request is not counted at all in the "Timings" tab – only for subsequent URLs entered. Either of both behaviours could be a bug, I don't know which one.)