My favorite mis-feature of these modals are the ones that announce something on page load, but they've also implemented "click outside modal to close it" so when you have any sort of muscle memory kicking in when you go to the website, you see a modal for 0.1s before it disappears as you click where you wanted to go.
On the opposite side, I'm also unreasonable frustrated when those modals appear and I cannot close it by clicking outside of it.
End results? Modals are horrible in most situations, especially when you want people to actually ingest some information.
There's another category of things that aren't really modals but they still prevent you from accessing some important user interface element because they draw on top of it even if they don't block out all UI elements.
Toasts in Windows are a good example -- often I am trying to use the tray but a toast pops up and I have to wait for the toast to clear or a toast pops up that makes me use the tray icons that it covers up if I want to deal with the situation. Of course on Windows there is the problem that clicking on a toast doesn't seem to ever do anything (like take you to the app that made the toast) and there is not a good mechanism to see the toast once it's past, etc.
In the case of Firefox I was particularly annoyed by little panels that floated above bookmark items on the chrome at the top of the page because, I dunno, there is something new I can do with my bookmarks, I guess. What I do know is that I wanted to click on something that was at the top of the web page and that stupid panel was in the way -- it wouldn't have stopped me from clicking on something else, but it's predictable that you're going to load a web page and frequently click on a link on a navbar at the very top.
On the opposite side, I'm also unreasonable frustrated when those modals appear and I cannot close it by clicking outside of it.
End results? Modals are horrible in most situations, especially when you want people to actually ingest some information.