I assume you refer to President Trump's controversial decision to move the US embassy to Jerusalem and hence further validate international recognition of Jerusalem as the capital of Israel.
Why are people mad about it? Simple: the people who are mad at it in some way refuse to recognize Jerusalem as Israel's capital. Some of these people are very hateful towards Israel and refuse to even recognize its existence as an independent state, so Trump's decision has naturally infuriated them, while others believe it simply to be a Palestinian city that Israel "stole" and that shouldn't be recognized as Israeli. Other angry people simply wish for Jerusalem to stay ambiguously recognized as a shared city, at least until one day a successful peace agreement between Israel and Palestine is reached. This last category of people therefore see President Trump's decision as reckless in the delicate situation of the Israel-Palestine conflict; it was an overwhelmingly pro-Israel move to make, one that deeply undermines the Palestinian Cause's objectives, to this day. Other countries have since decided to copy Trump's decision and this of course makes pro-Palestine people even more mad.
I personally do agree with Trump's decision, because Israel has fully controlled Jerusalem and has had it de facto as its capital for many decades now. "Palestine" isn't a functioning independent country and the reality of the situation is that Jerusalem does function as Israel's capital, so all has Trump has really done is recognized the simple reality of the situation. Governments should indeed work with the realities at hand, not with strange fantasies.
However, all that said, I will say that your reasoning is flawed.
Jerusalem doesn't "belong to the Jews" and Muslims certainly do have a claim to it in some way.
In thousands of years of Jerusalem's rich history, we have seen Jewish rule, Christian rule and Muslim rule. There exist holy sites for all 3 religions in Jerusalem. Jerusalem's population is mixed Jewish, Christian and Muslim, as well as other small faiths. To claim that "Jerusalem belongs to the Jews" is disrespectful and racist. Imagine someone saying "Warsaw belongs to the Christians" in WWII era Poland.
Jerusalem belongs to Israel. Israel's population is predominantly Jewish, but there exist Muslim and Christian Israelis too. Jerusalem is a city for all and should serve as a shining example of co-existence.
Whoever was "there first" is irrelevant. Thousands of years later we all live in the 21st century and were simply born here and now. Nobody needs to inherit guilt or any blood vendettas. Laws of land ownership, who legally owns which house and their civil liberties are far more important to the question of who gets to live in Jerusalem, rather than ancient blood vendettas.