You're supporting Google directly since 90% of Mozilla's revenue comes from their Google Search deal.
How am I supporting Google? I'm not giving Google any marketshare nor money. The money flows from Google to Mozilla and not the other way around. Google funds Mozilla because Mozilla still has a somewhat significant market-share. They could cut off the amount they give to Mozilla at any time if they wished, but the more people there are who stay with Firefox, the lesser chance of it happening there is. Your right in that it's not the best solution, but there aren't any other browsers that are less dependent on Google than Firefox, apart from Safari which is reliant on Apple.
In a lot of real world benchmarks Firefox is faster than other browsers (especially Chrome)
Vivaldi is heaps more customizable than Firefox in both UI and features.
Fair point, I know a lot of people like Vivaldi because of its UI. I personally didn't like the two tablist system, but I do see how it could be very appealing to a lot of people, especially those who open a lot of tabs.
Still, Firefox is really customizable with userChrome.css and the layout customization.
It doesn't. Librewolf does.
Firefox does have resist fingerprinting. It doesn't have it enabled by default, LibreWolf does. LibreWolf is basically Firefox with different default configuration, and Mozilla services disabled. You can achieve (almost?) everything that LibreWolf does with Arkenfox user.js, but for users who don't have the time or knowledge to bother with that, LibreWolf is a great option.
You are right. Firefox with the needed addons and fingerprinting protection might be slower than Chrome. But then if you would add the same amount of addons to Chrome, it would be also slower, so I don't know what you expect there.
But at the same time, a lot of things that Firefox offers (e.g. RFP, containers) do not exist for/in other browsers. Fingerprinting protection can be found in Brave, but as far as I know it's more lacking than RFP in Firefox, which also isn't complete.
And let's say that Firefox is slower than Chrome. Does that 300 millisecond difference really matter? Especially considering the above mentioned and more features you get with Firefox? There is literally no benefit that Chrome offers that Firefox doesn't.
If you say Vivaldi or Opera, although in my perspective their cons overweight their benefits (especially Opera), I can see your point, but there is literally no reason for anyone to run Chrome.
P.S. you need to realize how much of a threat Google is to the open web. Chrome is not W3 specs compliant, and Google also basically killed all indie browsers with Widevine, which is a proprietary DRM solution, which doesn't even work properly meaning it's only there for maintaining control over users, therefore it's a threat to users' freedom and privacy.