Firefox checks for an update the first time it is launched each day. Depending upon the updates server load at the time and the 'throttle' setting in use at the time, you may get the update the first time or it may come in a few days.

Typically each update is 'throttled' to 10% of 'contacts received' for the first few days after release and when no major problems are reported the 'throttle limit' is raised or eliminated in a few days time.

But if there are major problems discovered / reported, the update is turned off until a "chem-spill" fix can be made and tested before the new version is put on full un-throttled status again.

If you are impatient to get the new version immediately, you can use Help > About Firefox to get the new version and by-pass the "load sensor" and the "throttle"

that explain everything, thanks