"Con đom đóm"
Related film trivia:
I searched online for đom đóm to see if this was what the Joss Whedon series with Nathan Fillion is called in Vietnamese.
The results seem to indicate Tàu Đom Đóm is how it is named in Vietnamese. (Google translate gives tàu as ship.)
Another film title I recognised also came up, Mộ Đom Đóm, or Grave of the Fireflies.
Guessing, from the similarity of the word, phim is the Vietnamese for film.
As to fireflies, it is because such things are common in our neighbourhood in East / Southeast Asia.
In East Asia, fireflies have become a symbol of hard learning. There was someone in Jin dynasty (about 4th century) who was poor and could not afford the oil for night lamps, but he wanted to read books at nights. Then he found a pocket and collected fireflies therein, in order to use the firefly light. In Chinese there is an idiom nang ying (nang = pocket, here means to put something into a pocket, ying = firefly).
The story is also found in Japan. In Japan, the song Auld lang syne is sung with classical Japanese lyrics as hotaru no hikari (Light of fireflies), which is often sung at graduate ceremonies in Japan. There is also a magazine Keisetsu Jidai (Days of Fireflies and snow) in Japan, which aims for college entrance tests.
Given that this set of lessons is called 'Alphabet', I assume we are learning words like 'firefly' to give us an overview and practice with the different sounds and sound combinations in the language. And I really appreciate that duolingo lessons are structured this way. It will serve us well as we progress :)
Not sure if this will help you, but try paying attention to how the tone ends. The tone seems like it's falling in the beginning because it starts lower from the previous đom, but near the end it should be clear that it rises back up. Whereas if it was đòm, it will end with a very low pitch.
Also, in my experience (and I believe objectively), the sắc and huyền tones sound different depending on whether or not the word they are on ends in a plosive or not. Plosives are sounds like /t/, /p/, /k/ (see Wikipedia for more). If the word ends in a plosive, like 'sắc' itself, for example, the rise will be sharper and the word itself will sound shorter. In a word like (đom) đóm, on the other hand, the rise will be gentler and the word will sound longer.
The lessons didn't explain what the tones were, and while it gave a list of the diacritics, it didn't explain what they mean in terms of pronunciation. There's also a lack of audio in lots of these, so I didn't pick up on the fact that the grave and acute diacritics have intuitive meanings of pitch falling and rising.
I shouldn't be surprised--I always knew Duo was good for supplemental practice, not for being the primary teacher of a new language. But this seems so basic that I'm still a little surprised.
No wonder the icon for these lessons is a crying baby... the frustration is strong with this one! Personally i'd rather be learning less abstract words to begin with. Bread, bicycle and letter are all useful, firefly and Ferris Wheel are not something i'm likely to discuss much in general conversation.
I get that they are also trying to teach like spelled/pronounced words in this section, but without appropriate explanation of accents it's like grinding my brain against a blank sheet of paper and expecting it to remember stuff.