> 2019-10 says more than 76.0.0

Neither says anything useful. 2019-10 is no more actionable or informative than 76.0.0. That 2019-10 was released in october 2019 is meaningless information and only "more" in the "babble" sense.

2014-04 is absolutely actionable for a browser.

2014-04 is actionable for a lot of stuff - "unpatched" or "unmaintained"...

Well, I don't get an idea if my extension would break changing from version 2019-10 -> 2019-11 without reading the release notes.

I would know it there is a big chance of breakage if I moved from 76.0.0 -> v77.0.0 and I am reasonably certain it won't break if I moved from 76.0.0 -> 76.0.1

But if a website breaks, you can get more idea about why it happened by comparing your current browser's calendar version to the latest update. For example, you will know you are behind x years of updates. With semantic versioning, you don't know how old you have not been updating your installation.

Since there are more websites than extensions, the utilitarian approach would be to choose 2019-10 over 76.0.0.

One approach is proactive and the other reactive. Unless, one checks the release notes every 4 weeks... I would argue the utilitarian approach is exactly the other way. Verify and test if there is an API break with a major version change. Whether the browser is 2 years or 10 years old shouldn't make a difference.