Hello, everybody! Thanks for all of yours replies. First of all, I apologize for the delay. I’ve been kinda busy nowadays. 
Out of curiosity, why would you want to change the flag, if it’s making reference to the language? Both countries speak Castilian (Castellano) - idioms, aside. It would be like wanting to change the English flag to that of the U.S.A., Australia, Canada, etc. .
cassie, the Spanish spoken in Spain has nothing to do with the Spanish spoken in Argentina. That’s already obvious (seguramente ya conocerás los famosos casos de coger ≠ tomar, tontear ≠ joder, acabar ≠ correr, concha ≠ caparazón, y un largo etcétera). However, the difference between the text strings on the Mozilla Firefox’s es-AR and es-ES locales are bigger indeed, because this has to do with how people interpret technical texts. There’re a lot of cases, but let’s just take this one: add-on, in the es-AR locale it’s agregado while in the es-ES it’s complemento. Here’s another: the es-ES locale has The Book Of Mozilla quotations translated (run Mozilla Firefox or CometBird, enter about:mozilla at the address bar and hit the Enter key), the es-AR hasn’t, because sometimes is really hard to locale English geeky humor into Spanish (the quotation’s translations at es-ES aren’t good, after all).
Anyway, it’s not about the language only, cassie. The es-ES is set by default to use Spanish-localized web searching (like Google España) and they automatically prioritize locale content. For example, if you’re in Argentina using the es-ES default settings, most of the search results in Google, Yahoo! and others search engines will be somewhat related to Spain, ONLY THEN related to Argentina. It’s upside down if you’re in Spain using the es-AR default settings: the search engine’s results will focus on Argentina-localized content mainly. Of course, you always can change these preferences, but most people don’t know how to or they actually don’t care at all, even if they need to get a locale job or buy something near in the neighborhood. 
But I thought there was a separate Australian version, one that said, “G’day”, and called you “mate” at every opportunity. And a Canadian version that insists it’s really better than the American version, even if they look identical to everyone else? I would be shocked, shocked, I tell you, to learn that this isn’t true.
I’m having to assume that there’s a difference between the two, sort of like US and UK versions of English (though there’s not enough difference between the two in the context of a web browser, to confuse anyone. Since the different versions do exist, they should have the correct flags to go with them.
(That, or this “nationalism run amok” door should be firmly and decisively closed. Brazilian vs. Portuguese, Irish, Scottish, and how about my own Texan? A version that says, “y’all”? No. Just, no. Get rid of the flags, use the two-letter language code rather than go down that path.)
kluelos, in the specific case between es-AR and es-ES, yes, there’re big differences (I can’t tell about other locales because I can’t speak a lot of languages :P). The es-ES locale has almost any single word translated/localized, while the es-AR conserves a few words in English or some kind of “Spanglish Ad Hoc” (talking about the computer component, e.g., the Argentinean people say mouse, just like you, while the Spanish ones hate to call it like that, they use the word ratón, like the rodent).
Oh, Kluelos, don’t get me wrong, please. This has nothing to do with nationalism run amok, the locale versions have been created in order to help almost everyone to have a “home version” of a great web browser. In fact, one of the Mozilla Firefox’s strongest features is that, I mean, to have a LOCALE version, no matter where you call home. We’re not here to say hey, my locale version is better than yours!, no, man. If you’re American, I bet that you won’t feel very comfortable running a Great Britain English locale version or an Australian English one. You like most the en-US locale, right? That’s fine, as Great Britain people like its en-UK locale and Australian folks like its en-AU (this last one, in the case of Flock). This ain’t a locale war, this’s a global community, for God’s sake!
This has to do with CHANCES, OPPORTUNITIES TO EVERYONE. Let’s don’t waste it.
flag changed.
Sophia, you’re so courteous , always notifying everyone about anything.
Keep up that good work. Thanks again! I pay my respects to you. 
Finally, I’d like to thanks everyone about the comments here. I’m glad of hearing everyone speaking.
Best regards!