how to force an update with firefox 38
There is something wrong with your FireFox.

On MBA 10.9.5 with FF 38.01 there is About FireFox, that also shows new versions available.

Right now it is showing a 38.05 as new version that can be downloaded by clicking on it.

If that for some reason does not work for you you can manually update choosing your language first.

Check your settings for FF updates: