Download Firefox For All languages And Systems {web link} In your language.

This can also be a problem with the startup caches.

Try to start Firefox once with the -purgecaches command line switch to refresh the startup caches.
Make sure that Firefox is closed via "Firefox -> Exit/Quit Firefox".

• https://developer.mozilla.org/en-US/Command_Line_Options

Cor-El-- Starting Firefox from the terminal with -purgecache made no difference. FredMcD-- I had originally downloaded Firefox for English, and added the Czech language to the system. It seems a bit bizarre to have to uninstall it and install a special version for all languages. I am running version 77.0.1 (64 bits).

If you use the en-US Firefox version for English then you can install additional language packs for other language in the user interface.

• https://support.mozilla.org/en-US/kb/use-firefox-another-language

Note that for spell check you need a dictionary.
A language pack provides string translations for the user interface.

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

It seems a bit bizarre to have to uninstall it and install a special version for all languages.

The browser is in only one language that you choose. Not all languages.
After, you can add other languages if you want.

I had originally downloaded Firefox for English, and added the Czech language

Check the selected language and make sure of what is the default.

Before I came here to bother you with my problems, I checked the language selected, etc. I came here because the Firefox settings say that the default language is English, and yet, somehow, because I am in Prague, I get search results and user interface in Czech. ALL of that was already in my original post, so generic help suggestions are not helping.

   After I login, the interface is in English, but the next time I run FF it is in Czech again.  Is there a way of making it stick without logging in?

Is this about the language for the user interface or for the language used for web pages ?

Both have their own setting in "Options/Preferences -> General -> Languages", so do not get confused.

• https://support.mozilla.org/en-US/kb/use-firefox-another-language
• https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages

Can you confirm whether your problem is about either of these two (or both) ?

If you have a problem to keep the language for the user interface after a restart then this is likely a problem with the Firefox startup cache like I posted above.

If this is also a problem with the language for web pages then you might have a problem with external cleanup software Advanced SystemCare or CCleaner and you need to check their settings.

My students say that I am good at explaining things, so my utter failure to communicate my problem here is puzzling. the SUBJECT of my post says: USER INTERFACE AND SEARCH RESULTS are in the wrong language. How can that be misunderstood? The pot says that I am running Linux mint/XFCE, so there i no ccleaner issue, since ccleaner is for Windows.

   Perhaps I should rephrase my question:

It seems that if I log in to my Firefox account, I get English (for both user interface and search results), which is what I want. Is there a way for FF to ALWAYS open by default into my FF account? I normally open it by clicking on its icon in the panel, or by launching it from the terminal. I know I can create a launcher for it, which-- I assume-- could include the direction to my profile, and perhaps even log in to my FF account, but this seems a lot of effort for a trivial result, that should be easily set in the preferences.

  BTW-- right now in the preferences Czech is not mentioned anywhere-- all the languages are set to English.
  I appreciate the effort and time yu are putting into helping me solve this rather annoying misbehavior of an otherwise good browser.

I finally solved this irritating problem in the following way: I created a profile, where I specified the home page, etc. Now when I open Firefox, I can understand what I see on the screen, since it is in English, and it stays in English. It was annoying, but it's OK now.

That was very good work. Well Done.