Even if it is a "real emergency," modal dialogs still don't make sense because people are so annoyed by them, they don't read them. If you really, really want the user to read something, don't put it in a modal that pops up over the thing they actually want to do. They're pretty much horrible UX for any conceivable use case.
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.
There are certain applications that I'm frequently using to resolve a problem (say I just found out a bill is overdue) where I really feel under the gun and I find it astonishingly annoying to have to close a large number of dialogs telling me about new features.
A human being with some empathy might realize that you're initially in a state where you're not receptive to a message and later realize you are.
If I got a popup advertising a new feature after I completed a task I'd be a lot more receptive to it, particularly in that I'd be feeling the glow of having completed a task, being satisfied with the product, and not feeling so pressured, having some headspace to learn about a new feature.
So is the appearance of any modal window something that justifies filing tickets with Firefox?
It doesn't sound like that's PaulHoule's motivation anyway — he knows that these tickets are not going to be fixed, just closed — the point is to "pour just a little bit of sand in the gears".
If usability tickets are closed because the company doesn't want to bother, then maybe these gears deserve to have sand put in them.
I generally approve of subversive actions which are naturally damaging if and exactly if the accusation they are based on is true.
That is, the logic is something like "well, either it gets fixed, in which case it's a victory for good, or they're hypocrites who don't really care, in which case 1. it wastes their time and 2. they deserve to have their time wasted."
I dunno. I don't personally report many bugs in public software...
But reporting real problems because you are annoyed that problems keep appearing and don't get fixed doesn't look like a societal hurting behavior to me. It does look like personally hurting, but antagonizing the author because of this is a real societal hurting behavior.
Just because a modal window exists doesn't mean it's bad for accessibility.