Zoroastrianism is a bit difficult. We just don't know much about the earliest formations of the religion. It most likely began with a people called the Aryans, which is where Hinduism also develops from. There are a lot of connections between Hinduism and Zoroastrianism, including a lot of similarity in the languages that composed their earliest writings. The main problem though is that Zoroastrianism is understudied, so definitive information is harder (especially as there is so much misinformation from people claiming to practice the religion, but really they are just taking bits and pieces of various religions and throwing them all together). A great source of Zoroastrianism, if you want to dive more into it is an article by Michael Stausberg (who is probably the foremost expert on the religion) titled On the State and Prospects of the Study of Zoroastrianism. Another great source for an over view of the history is Zoroastrians by Mary Boyce, which, even though is was published in 1979, is still often the go to textbook.

So, when it comes to what Zoroastrianists thought of polytheism, we can only look at a somewhat later date. Before we jump in there though, there is some debate as to whether Zoroastrianism was truly monotheistic, or if they were polytheists, or practiced a form of dualism, and then only later, more in the modern period, decided to rewrite the narrative. A great source on this Solomon Nigosian's work, Zoroastrian Faith: Tradition and Modern Research. A conclusion that he reaches is that there is a lot of mystery in the Zoroastrian belief, and its not always straight forward, so there are a lot of differing views. It could be that they are more henotheistic, but some form of worship of a single divine figure was held.

Most likely, and Francois de Blois, in an article titled Dualism and Iranian Christian Traditions, shows how this worked, is that there are stages of development within the religion. It goes from more polytheistic to dropping even the dualistic view, and becoming more of a straight monotheism.

So to the question. While we can't look definitely at the development, we can see how Persian rulers, who were Zoroastrianists, treated others. Zoroastrianism really dominated a couple of Persian dyansties, such as the Archaemenias, as well as the Parthian and Sassanian Empires, and would only be overthrown during the Muslim conquest in the seventh century CE.

One of the famous Persian leaders, Cyrus the Great, was himself a devout Zoroastrianist. From what we can gather about him, he was quite a tolerant ruler who took to heart to aspect of Zoroastrian law of asha, which is truth and righteousness. It could have been this that made him choose not to force the religion on others, and instead be quite tolerant of other beliefs. This really followed through the Archaemenian empire with other figures such as Darius the Great.

Later on, after the conquest by Alexander the Great, and then the eventual return to a Persian empire under the Sasanians, things did change. The Sasanians were far less tolerant. It's really with the Sasanians, who took power in 224 CE, that Zoroastrianism really changed to be far less tolerant. The religion also underwent large reforms at that time too.

As a whole then, from the earliest time period that we know a good deal about Zoroastrianism, that others were polytheistic wasn't a problem. Most likely, Zoroastrainism began as a polytheistic religion, that became henotheistic, in that they had one god, but accepted that other gods existed, and eventually moved to a dualistic view, which was then adapted into straight monotheism. If we look at Cyrus the Great, he even promoted polytheism a bit by restoring various gods to different cultures. This could suggest that he was a henotheist, or it simply was a good political move. Later on, the religion would reject the idea of other gods, but they still continued to be pretty tolerant.

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Moving to Judaism, where we have a lot more information. Mark S. Smith is one of the leaders in the field of early Judaism, and the formation of the idea of God. A great work of his is God in Translation: Deities in Cross-Cultural Discourse in the Biblical World. Smith really traces the the development of what he calls the "one-god worldview" of Israel. It starts as a polytheism, more of a henotheistic view, which morphs into monolatry, the worship of only one god, to monotheism, where there is only one god.

One point that really sticks out with Smith is the argument that Israel, while still moving towards monotheism, as they were still a polytheistic religion, was much more violent and intolerant. The idea wasn't that all other gods don't exist, but instead that everyone else was worshiping false gods and the Israelites were going to show just how powerful their god was. And that was often through violence.

The change to more tolerance, for Judaism, came as they become more monotheistic. Putting this all more in perspective, John H Walton has a work titled Ancient Near Eastern Thought and the Old Testament: Introducing the Conceptual World of the Hebrew Bible. What Walton points out is that Israel really differentiated their god from the surrounding gods. There is no origin story for Yahweh. Many of the qualifiers for Yahweh are eventually stripped away (qualifiers that were used for gods in generals). There is a conscious move to separate Yahweh, and in doing so, the Hebrews, from their surroundings.

Part of this may be that Israel was under almost constant attack. Donald Redford, in his boo, Egypt, Canaan and Israel in Ancient Times looks at how Israel was really between a rock and a hard place as Palestine, and the Middle East, was often squashed in between massive nations fighting for control. Finding a way to separate themselves, and really preserve themselves was quite important.

Once Israel really does finally formulate the their idea of monotheism, while they did reject other gods pretty much outright, they also became more tolerant of other views. That was partially because they saw themselves as set apart from others.