Things do get fixed, but it is a big project with not a huge amount of staff.
The problems mount up over the years while it seems that meanwhile, Mozilla has a lot of frills and luxuries on the budget as an organization, when the core product is generally lacking in terms of compliance and quality.
It's frustrating, because I want to like Mozilla, but their browser is the most work to develop for, and the least fun to use (in my case in particular, since Firefox is generally worse on Linux than on Windows).
Added: I'm also frustrated that when I share my personal experience of the process with Mozilla, people won't have it (not talking about _yoasif here, good work on those successful reports!). I get that it's your baby, that you have a personal relationship with Mozilla that makes you want to defend it, but if you love it, you must allow it to face honest criticism, even if at the end of the day you'll choose to ignore it.