If a person dies by burning is he regarded as a martyr?

The one who dies by burning is a martyr, because of the
report narrated by Ahmad (23804), Abu Dawood (3111) and al-Nasaa’i (1846)
from Jaabir ibn ‘Ateek (may Allah be pleased with him), according to which
the Messenger of Allah (blessings and peace of Allah be upon him) said:
What do you regard as martyrdom?” They said: Being killed for the sake of
Allaah. The Messenger of Allaah (S) said: “Martyrdom is seven things
besides being killed for the sake of Allaah. The one who dies of the
plague is a martyr, the one who drowns is a martyr, the one who dies of
pleurisy is a martyr, the one who dies of a stomach disease is a martyr,
the one who is burned to death is a martyr, the one who dies beneath a
collapsed building is a martyr, and the woman who dies in pregnancy is a
martyr.”  

Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

It says in ‘Awn al-Ma‘bood Sharh Sunan Abi Dawood: 

Al-Mat‘oon is the one who dies of the plague. The one who
drowns is a martyr if his journey was for a permissible purpose. The one who
dies of pleurisy – this refers to pain in a person’s side, which calmed down
and the pain ceased at the time of death; one of its signs is pain under the
ribs and difficulty in breathing, along with constant fever and coughing. It
is more common in women, according to al-Qaari. Al-mabtoon (the one who dies
of a stomach disease) refers to diarrhoea or dropsy or pain in the stomach.
The one who is burned to death refers to one who dies in a fire. The one who
dies beneath a collapsed building – this refers to falling walls and the
like. The woman who dies in pregnancy – al-Khattaabi said: what this means
is that she dies with a child in her womb. End quote. 

And he said at the end: i.e., she dies with a child in her
womb. And it was said: this phrase [in Arabic] may refer to the woman who
dies a virgin. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: If a man of the civil defence dies as a result of being burned with
fire when he was trying to put it out, is he regarded as a martyr? 

He replied: Firstly, I say: everyone who dies by fire and is
a Muslim is one of the martyrs, because the Prophet (blessings and peace of
Allah be upon him) said: “the one who is burned to death is a martyr.” But
we cannot say that So and so is a martyr because he died by burning, because
we do not know. Rather we stay in general terms that everyone who dies by
burning is a martyr, and if he was a firefighter then he will attain a
greater reward, because the one who died seeking to put out a fire combined
two things: he was burned to death and he was protecting his brothers. So in
fact he has attained two rewards: the reward for protecting his Muslim
brothers and the reward of martyrdom for being burned to death. But you
should note that we do not testify for any particular individual, meaning
for example, that if a person is burned to death in front of us, we say “the
one who is burned to death is a martyr,” but we do not say “this man is a
martyr.” Al-Bukhaari (may Allah have mercy on him) referred to this issue in
his Saheeh and said: “Chapter: It cannot be said that So and so is a
martyr.” Then he quoted as evidence for that the words of the Prophet
(blessings and peace of Allah be upon him): “ … wounded for the sake of
Allah; Allah knows best who is wounded for His sake.” He said: Allah knows
best; therefore if Allah knows who is wounded for His sake, then we cannot
testify for anyone; rather we say in general terms that the one who is
killed for the sake of Allah is a martyr. Thus we differentiate between
speaking in general terms and speaking in specific terms. 

End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen
(25/448) 

And Allah knows best.