You're confusing a specific group and a specific situation with a general statement of fact. In your first example, specific firefighters arrived to help at a specific accident site. In the second, firefighters IN GENERAL are often the first to arrive at ANY accident. In the other examples, all sea turtles are protected all the time by the government, so we don't need a definite article. I'm not clear why you state that "... species of animals... are used with the definite article." I can't think of any examples that support this (we say "mammals" and "birds" and "fish," we don't add "the" to them unless we are referring to a particular group of them).

Olga, 1) The firefighters were the first to arrive at the scene of the accident. - If a modifying word, phrase, or clause follows the noun and makes it clear which specific person or thing you are referring to, then you should use the definite article. The modifier in this case would be "the first". It specifies who came first: the firefighters. Also, the use of the definite article with "the accident" tells you that the sentence refers to a specific accident and not accidents in general. 2) Firefighters are often the first to arrive at the scene of an accident. Ø + plural noun. When you are making a generalization you can use the plural form with countable nouns. The use of the indefinite article with "an accident" indicates that we are not talking about a specific accident but about accidents in general. The use of the present tense here also indicates a generalization 3) Sea turtles are protected by the government. Ø + plural noun. This is also a general statement. If you were referring to a species you would write: The sea turtle is protected by the government. The + singular noun is the formal way to make a generalization.

Olga, you are right about species. We can say "The sea turtles are protected by the government" or "The sea turtle is protected by the government" or "Sea turtles are protected by the government." All of these mean "All sea turtles." English is strange, isn't it?

You know my advice here is don't be so serious about "the", you are not talking about "an" here, actually. Denis is absolutely spot on but it wouldn't sound too bad to add "the" to your example 2. Excuse being that, you can, actually, argue that when you refer to firefighters, they do, always belong to some our so-called "specific group" of things. They are never peddlers, they are never scalpers, but firefighters. And that makes them "specific" from other jobs. But you know what to choose in a formal test situation. And if you know American slang culture well enough, you would find "the" almost appears in most of the sexist slang terms, where according to the books, you are not supposed to apply it. And in daily speech people would feel comfortable to always say, "the Americans are stupid", "the Chinese always do that", even though they are not talking about the Chinese in a group, his colleagues, or something, but like you say, "general phenomenon", in which case the "the" is apparently redundant by THE book.