tl;dr: A notable marketshare of multiple browser components and browsers must exist in order to properly ensure/maintain truly open web standards.

It is important that Firefox and its components like Gecko and Spidermonkey to exist as well as maintain a notable marketshare. Likewise, it is important for WebKit and its components to exist and maintain a notable marketshare. The same is true for any other browser/rendering/JavaScript engines.

While it is great that we have so many non-Google Chrome alternatives like Chromium, Edge, Vivaldi, etc., they all use the same or very similar engines. This means that they all display and interact with websites nearly identically.

When Google decides certain implementation/interpretation of web standards, formats, behavior, etc. should be included in Google Chrome (and consequently all Chromium based browsers), then the majority marketshare of web browsers will behave that way. If the Chrome/Chromium based browsers reaches a nearly unanimous browser marketshare, then Google can either ignore any/all open web standards, force their will in deciding/implementing new open web standards, or even become the defacto open web standard.

When any one entity has that much control over the open web standards, then the web standards are no longer truly "open" and in this case becomes "Google's web standards". In some (or maybe even many) cases, this may be fine. However, we saw with Internet Explorer in the past this is not something that the market should allow. We are seeing evidence that we shouldn't allow Google to have this much influence with things like the adoption of JPEG XL or implementation of FLoC.

With three or more browser engines, rendering engines, and browsers with notable marketshares, web developers are forced to develop in adherence to the accepted open web standards. With enough marketshare spread across those engines/browsers, the various engines/browsers are incentivized to maintain compatibility with open web standards. As long as the open web standards are designed and maintained without overt influence by a single or few entities and the open standards are actively used, then the best interest of the collective of all internet users is best served.

Otherwise, the best interest of a few entities (in this case Google) is best served.

You don't even make sense. Themes slowing down the browser, what?? That doesn't happen.

Ff is faster for me

Chromium is a dead end, I use Firefox because it respects my privacy but also because it works better.

Hmmm. Hard to get over that dislike of an interface. I have found one plugin killed Firefox performance for me but I'm generally happy with the layout so haven't messed with it.

On how important it is: I used Chromium for a while but went back to Firefox because I read someone somewhere say "If you don't use Firefox, there eventually wont be a Firefox to use" and that was enough for me to switch back.

If you're donating to Mozilla though, I guess they're more than happy with that even if you're not using the product.

Use what you like and don’t give a fuck about strangers on the internet deem the holy grail of browsers.

It matters as much as you want it to.

Out of interest what part of the UI don't you like? You can drag and drop pretty much any button and component where ever you want and you can use the Firefox colours website to apply any colour scheme you want. This is all core browser features so no performance affects.

What is it that a theme is then adding?

Well, if Firefox does become unusable for you, then I don't think you would be at fault for switching to a Chromium-based browser, like Vivaldi. Usability is an important aspect to consider.

Have you considered using a Firefox fork, like LibreWolf?

Remove all your plug-ins and run it standalone. Firefox doesn't drag ass.

I prefer supporting browser alternatives as opposed to supporting Google's monopoly of web browsing

Whatever you've done to UI must be some atrocity as I do not experience issues with FF. You've never specified which FF extension you've used that had slowed down your browser.

Chrome (and by extension) Chromium and all derivative browsers are Google's lever to truly control and shape internet to their liking. Multiple people said it already.

Personally I find Chromium UI very cumbersome and dislike it a lot. Which is to say we all have our own preferences for UI.

In your case you'd have to weigh your repulsion with available performant FF UIs vs future of internet and choose which decision can you really live with.

Use what you prefer, except brave, they can go fuck themselves.

Firefox works great for me, but I have OP hardware.

using firefox on Linux with i5 6th gen, ddr3 32gb.

have over 100 tabs. most are suspended. cannot say it is slow.

edit: running with NVMe

Here you go

Use whatever browser works best for you. It is kind of dumb to stick to something you do not like just because of ideology.

That said both Firefox and Chromium based browsers have pros and cons, there is no best one. My primary browser at home is Firefox, at work I use Edge, I like both. Some time ago tried to fully move from Firefox to Vivaldi, but went back due to couple things that I preferred on FF.

Your main problem is probably 6GB physical memory. At least on Windows 10+ I would not be comfortable with less than 16GB. Hope you at least have SSD.

Firefox is faster than Chrome for me. Theme isn't an issue for me, but the thing that took me a long time to get where I like it, is getting rid of all the buttons and UI elements and plugin icons that I didn't want to see all the time to get things streamlined down to where I like them. It did take me a couple months to get everything how I like it, but now when I use chrome, it seems clunky.

Trying to switch tools takes a while to get right and to re-learn. There are some things I don't like about Firefox still, but more I don't like about Chrome.

I use Firefox because Chrome screws up my task bar icons too often. I have to use a PWA extension for Firefox to get that functionality, but once set up it just works.