Even posts I should be professionally partial to, like those about UI design, rub me of the wrong way. It's often full of "creating delightful experiences" and "streamlining a seamless flow".
> I observed the change in tone in Mozilla blog posts, from experts talking about their craft to marketspeak.
I think I can explain what you're seeing here. The original blog.mozilla.org was run very much like the old "MSDN blogs" from Microsoft (think Raymond Chen's The Old New Thing.)
When I started at Mozilla a decade ago, my manager told me to file a request for my own space on blogs.mozilla.org -- which was denied. I was told that this was because blog.mozilla.org was being refocused as the "official" Mozilla blog.
People who already had accounts on there were grandfathered in, which is why some people (Nick Nethercote's blog come to mind) still had blogs on there. Obviously as those developers moved on, those employee blogs have gradually died off.
These days, if you want technical content, you'll need to look at hacks.mozilla.org for "officially sanctioned" pieces, or look at the blogs that publish to planet.mozilla.org for developer blogs (though much like blogging in general, there is much less traffic on there compared to a decade ago).
TL;DR: The focus of blog.mozilla.org is primarily corporate and marketing at this point.