A new version is scheduled to be released every 6 weeks. Sometimes this is delayed over holidays or for other reasons.

So that would be: 27.0

When there is an emergency fix for Firefox, this usually is designated with a further decimal point:

So that would be: 27.0.1

I don't know why that isn't 27.1, but either way is fine with me.

if emergency fix is released then it can release 27.1 why they are updating as version 27.0.1

Here is another data point: there is an extended support release of Firefox in addition to the regular release. Its version number is incremented when it gets each 6-week critical bug fix: 24.0esr, 24.1.0esr, 24.2.0esr, 24.3.0esr. When there is an emergency fix, the 3rd place is used, so for example, 24.1.1esr.