The answer depends on what language you are asking about.
In English, "...have been..." is a statement about the present as well as the past. It talks not only about what happened, but also about what the state of affairs is now. For instance, in English, "The software has been installed" means "The software is in a state of having-been-installed" - or, to be less eccentric about it, it means that both (1) "The software was installed" and (2) "The software is still installed". On the other hand, in English, "...was..." is a statement purely about the past. It says nothing about the present.
In American, this distinction is rarely made. The form "...have been..." is rarely used, and "...was..." usually replaces it. This often causes confusion. For instance, when an American-speaking computer pops up a message saying "The software was installed", an English-speaker will think "Why did it not say the software has been installed? Does it mean that the software was installed but then something went wrong afterwards?".
So in your case, as a foreigner, the best thing is to learn the more precise distinction - thus, using "have been" if you have been fired and are still fired and it happened recently, and "was" if you are talking about a more distant past, or you already have another job. This is correct English, and speakers of American will understand it without thinking it strange.
(Interestingly, there is exactly the same Atlantic split in Spanish: in Europe, me han despedido means it was recent and I haven't got another job since, while me dispidieron means it was further in the past or I do have another job; in Latin America, they use me dispidieron for everything).