Fx has had many man-hours put into it, attempting to reduce their technical debt and improve the efficiency of maintenance work. They know what's still wrong in the browser, and they're working on it, but progress is slow. There's a reason that Mozilla's working on Rust, and it's not just for S&G.
MySQL is so bad that when MariaDB happened, several long-standing architectural issues were cleaned up. Maria's got a lot of improvements that were made in response to MySQL's community not making those improvements.
Linux... I don't understand why you put Linux on this list. The only people complaining about Linux architecture are:
* Tanenbaum followers who believe that microkernels will save us all
* Binary driver authors who want Linux to be more amenable to their evil schemes
* C++ weenies who are offended by Linux's refusal to adopt C++ as other kernels have done
* C hackers who don't like the brace style of Linux
All of these groups have axes to grind; they're not approaching things from some position of "this code is not performant enough" or "if only more things were supported."
WP and Wordpress picked the wrong implementation language. It happens. Migrating them off of PHP is non-trivial. We already know all of this. It's not that we think everything is going to fail; it's more that we have seen Wordpress get hacked so many times over the years and we're sick and tired of it.
Edit: Attempt to salvage list formatting.