The list under

Preferences --> General --> Language and Appearance --> Language

is sent to web pages with your request so they can respond in your preferred language. That list does not need to match what you do with language packs, since it has a very different purpose.

Why does this list include languages I never selected and leaves out those that I did select? How can it help me if a web page responds to a request of mine using South African English, a language I am totally unfamiliar with? instead of South African English that was apparently selected at random by Firefox, the choice could have been Sanskrit or Swahili with rather suboptimal results. How can I tell Firefox that I want web pages to respond to me either in US English (my first choice) or otherwise in Greek?

Iason2004 said

How can I tell Firefox that I want web pages to respond to me either in US English (my first choice) or otherwise in Greek?

You could edit the list on the Preferences page. You said you currently have this, so you may already be set up:

(1) English/United States [en-us] (2) Greek [el] (3) English [en]

The third one is not regionally specified, so the site gets to decide what it thinks that means. I assume it's the rare site that distinguishes one flavo[u]r of English from another.

I am afraid I got mixed up when I posted my previous question. The set of languages under Preferences --> General --> Language and Appearance --> Language is indeed

(1) English/United States [en-us] (2) Greek [el] (3) English [en]

as was set by me during installation. I understand now that this info is communicated by Firefox to sites I visit and they respond accordingly in US English, Greek, or, failing the above, some other variant of English as they may see fit. So far so good.

Now we come to the display found under Add-ons --> Languages which as of now reads

English (GB) Language Pack English (GB) Language Pack

English (South Africa) Language Pack Adds South African Language Support to the Firefox browser. All interfaces will be translated into...

Ελληνικά Language Pack Greek (GR) Language Pack

Well, what is happening here? How did these language packs find their way here? Certainly they were not selected by me. What purpose do they serve? On what criteria did Firefox select them if not at random? Why did Firefox become unable to connect to any website when I disabled the GB and South African English language packs? Why is there no way to replace these language packs with the presumably more relevant US English and non-regionally-specified English language packs?

Could you please shed some light on the above?

If you are running the U.S. English installation of Firefox, then you do not need a U.S. English language pack.

As for the others, I have no idea how you got those or why they affect websites. Hopefully someone more multilingual can shed some light. Until then, maybe check out Use Firefox in another language.

P.S. If you install Firefox from your distribution instead of directly from Mozilla, there may be discrepancies from our understanding of what is included with Firefox.

That is probably how Ubuntu packs its own branded Firefox version that is offered via the repositories and possibly automatically installs language packs. You can see the same with installed dictionaries if you check the Language sub menu if you right-click in a text area.

Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

• https://www.mozilla.org/en-US/firefox/all/

jscher2000 said

I have no idea how you got those or why they affect websites. Hopefully someone more multilingual can shed some light. Until then, maybe check out Use Firefox in another language.

Prophetic words. The previous post by cor-el did just this. Meanwhile, I found the link provided about Firefox in other languages with language packs extremely useful not only within the confines of the present thread but within the wider context of using additional languages with Firefox. Thank you very much.

P.S. If you install Firefox from your distribution instead of directly from Mozilla, there may be discrepancies from our understanding of what is included with Firefox.

Oh yes! There can be and how! As I mentioned in my first post, I landed into this problem by installing Ubuntu 16.04.3 which comes with Firefox 56.0 (32-bit) Mozilla Firefox for Ubuntu Canonical-1.0. Yes, this version of Firefox did not come directly from Mozilla but was retrofitted by Canonical for reasons that are their own. Once this version of Firefox is installed, future updates are similarly handled by Canonical which continues to interpose itself between Mozilla and end user of Firefox. Yes, this does create a scenario for discrepancies as jscher2000 mentioned.

Furthermore,

cor-el said

That is probably how Ubuntu packs its own branded Firefox version that is offered via the repositories and possibly automatically installs language packs.

Yes, that seems to be the problem. As for the odd choice of language packs, Wikipedia does provide some useful clues, i.e,

Wikipedia [said]

Canonical Ltd. is a UK-based privately held computer software company founded and funded by South African entrepreneur Mark Shuttleworth to market commercial support and related services for Ubuntu and related projects.

So here is strong evidence concerning what motivated the choice of UK English and South African English language packs for the Ubuntu version of Firefox.

Finally, I did not check cor-el's suggestion to

Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

because I find it too troublesome for my skills to install Firefox in Ubuntu directly from Mozilla. In addition, a live Puppy Linux CD I use has Firefox ESR 37.x.x installed, which even if upgraded to the latest 52.4.0 version does not display any language packs under Tools --> Add-ons, a wise choice I would say as it prevents threads like this one from ever materializing thus saving time for more productive activities.

In closing, I thank both jscher200 and cor-el for helping me make some sense out of all this confusion.