This aligns to my understanding of the usage.
Bitterness comes from a kind of unhappy acceptance. You’re bitter because something sucks and it’s unlikely to change. You can grumble, but it’s dark and sullen.
Saltiness, to me, is more vocal. You’re still in the “make sure everyone knows you’re unhappy about the thing.” You may or may not believe your complaining will fix the problem but you haven’t descended into bitterness yet.
They’re related shades of unhappiness, but they’re not synonymous.
Surprised we haven’t mentioned ‘sour’ in this thread yet. Obviously not the same as ‘bitter’ or ‘salty’ (which are not quite the same as each other), but it seems related or at least slightly overlapping as a term to describe mood or personality.
And of course we can be sweet-natured, or sweet on somebody.
But this the traditional western four-flavour model. Not all that recent science tells us there’s a fifth flavour. So how long before we can describe someone as being “ a little umami”?
I must be either old-fashioned or in a dream world, or both, but I only know “salty” as description of language - swearing and other vulgarities - presumably associated with sailors. I suppose that would arise from irritation and anger.
The UK is sort of on the way there (Marmite is indeed very salty, but being yeast-based is also rather umami):
There are also other flavour-related terms including ‘spicy’ (eg ‘wow, that’s kind of a spicy take’). I can’t for the moment think of others but I bet they’re out there.
Cloying and effervescent.
I know it’s a hopeless rearguard action and language is determined by usage etc, but that one still does really GRATE on my ears…
To me, “bitter” implies something more long-lasting than “salty” - I might say that someone got salty because I made a joke about them , but it would be odd to say the person was bitter after a single joke.
Not to pile on but here’s another up/down conundrum:
It’s summer. It’s hot inside your house. You wish it was cooler. So do you “turn up the air conditioner”, or “turn down the air conditioner”? 
This is an Americanism, we in the Queen’s English speaking world use “salty” to refer to either the condiment, or to older sailors.
Not to apparently disgruntled people.
‘Salty’ started as a taunt in online gaming and spread there first as I recall.
It probably should be “turn down the thermostat” and “turn up the air conditioner,” but, I admit, in the moment I probably mix it all up.
D’oh. Shows my age. I forgot about that 
.
FWIW the reason I was thinking about this is because a friend of mine was asking me about a different word, this time in Spanish rather than English, and how it’s use seems to have changed in Spanish in recent years.
It amused me that it was the first link in the ‘Suggested’ list, I couldn’t resist.
You’re not the only one. There’s a thread here about starting a thread and being informed that this is the second time the user has started the exact same thread, followed by wondering why the software doesn’t do a better job warning them they’re repeating themselves.
No, it’s a generational thing. My kids and their friends use it exactly as a synonym for bitter. Or sour. All 3 just mean ‘a negative attitude that makes you 
’. I just checked with the 20 y.o.
He’s hurting inside about the loss.
He will snap at you if you bring it up.
I used to work in an office that was frequented by Jerry Jones. I knew him socially. He was both bitter and salty always… and an asshole.
After a few instances of such confusion, my wife has settled on “Make it colder!”
As for “on accident,” I would point out that it is precisely parallel to its opposite, “on purpose,” so maybe it’s more logical than “by accident.”
Heh! I say that all the time.
The thermostat confounds me. I ask my family to say “it’s too cold or it’s too warm” to be clear.
Regardless, up and down can have ominous repercussions in my world.