I think more appropriate term for “firm” would be “company”.
More appropriate in which sense? This is correct English.
It's certainly correct, but it's just more natural to say "company" instead of "firm."
When you are a lawyer or work at a lawyer's office, you say I work at a law firm. Honestly, that's the only time I know of when English speakers use firm.
Check https://www.google.com/search?q=%22working%20for%20the%20firm%22&tbm=bks&lr=lang_en then
just to make sure please: does "pro" nemeckou govern instrumental case? thanks
pro requires accusative
Take a look at the noun "firma" and see what it looks like in various cases and you'll find which case "firmu" is. https://en.wiktionary.org/wiki/firma#Czech (plus, practically all nouns ending in "-a" behave the same way)
thank you very for in depth answer ! Thank you both