A while back Google's Chrome took the number one spot in the browser market share worldwide. Chrome is definitely a very good browser, and it has been favoured by web developers for quite some time. There might be arguments made that it might be the best browser on the market. At least it seems to be the fastest, and because of the fact that everyone develops on chrome, it's probably the one that most websites work on.
But here is why you probably shouldn't use it:
Competition
Competition is always good. If there is no competition in the browser market, there won't really be an incentive to make new features, or even improve security in the dominant browser. I do realise that Chromes market share is a lot smaller than Internet Explorers was at it's peak. But there is a key difference, IE has always been a hated browser, developers and geeks have mostly used alternative browsers, and because it's those very people who are pushing for advancements in web standards, the market share of IE was never quite as relevant. Also, if there is only one browser on the market, the web standards won't be very important at all. What would matter is if it works on the dominant browser. This will make it very hard for new browsers to enter the market, even if that market is not one competing with desktop Chrome. For instance some new type of mobile operating system, or browser.
Conflict of Interest
This one is the all important one. If I use a browser on my computer, I wish for that browser to work for me. What I mean is that it's my computer, and I should control the software running on it. Things outside my computer, like websites I visit, are out of my control, and might be considered potentially hostile. Even if you trust the website in itself, there might be hostile ads, or the website might even be compromised.
When this happens I prefer to have the brower vendors interest mostly align with mine, whatever my interest as a user of the software happens to be. This is not really the case with Chrome. Google is an ad based company, and a content provider. This means that it's in their best interest to show you as many ads as possible, and to protect their intellectual property. Take one simple example: If you go to YouTube with your browser. You might want to skip some annoying ads, or you might want to download some videos from the site. Sure, both of these things are against their terms of service, and may even be considered morally wrong. But in most places they are probably legal to do. Many countries allow copying of content for your own use, or for research purposes. And the ad skipping is probably good for the general level of mental health. And in many areas there is no paid subscription to get rid of ads on YouTube.
But when it's the same company who are making your browser and the content you are watching, you can probably assume that the browser will work in the interest of the content company, not you. For instance the YouTube video downloaders are not allowed on the chrome store, even though they don't have any similar rules when it comes to extensions affecting competitors sites.
Why Firefox?
This might mostly be a process of elimination. Don't get me wrong, Firefox is a really good browser as well, but it might also be the only real alternative to Chrome.
Microsoft Edge is a somewhat promising browser, but it is missing a lot in form of features and extensions, and it is horribly inefficient. It also runs just on Windows.
Apples Safari is not a good browser. I'm sorry if that offends anyone, but it's old and creaky, and by that I mean that it haven't been improved on in the same way as Firefox or Chrome has. It plays fast and loose with web standards. Of the big 5 (Chrome, IE, Firefox, Safari and Edge) it's by far the least standards compliant. It also only runs on Apple devices.
And we shouldn't talk about IE, there is no reason to use that one anymore.
But as I said, Firefox is actually a really good browser. It was getting a bit iffy there at one point, but development has really picked up steam lately. It's probably the most stable browser, and although not always quite as fast as Chrome, it can still be quite snappy. For a long time the web development options were quite a bit behind Chrome. There were add-ons that improved the situation, but Chrome had awesome developer tools built in pretty much from the start. However, in my opinion Firefox might have surpassed Chrome with their built-in developer options. And they also have a developer edition with even more tools built in.
Firefox is also introducing a lot of new features all of the time. The biggest ones lately is probably the multi-process support for increased performance, as well as the user context containers that are in testing at the moment. The latter is an awesome privacy and security feature, where you can have separate containers with separate cookies and storage options for Personal things, or Work thing, or basically whatever you want. For instance, if you create a separate Facebook container, and always visit Facebook only in that container, it is pretty much impossible for Facebook to track you on third party sites.
Conclusion
At the end of the day, you can use any browser you want. I just wanted to highlight some points regarding using different browsers. We shouldn't just blindly use one browser because everyone is using it. I find it especially disconcerting that basically all web developers use Chrome exclusively (if you don't count basic testing). I've run across e-commerce sites with probably millions in sales per month that doesn't work on anything but Chrome. So use whatever you want, but give the alternatives a try. Even if you end up on Safari, that would be your choice, not someone else's.