In my experience, two folders were left with Firefox. 1 with your own profile and cache and 1 for installation.
I have looked at the whole installation currently which is 347MB with a size of more than two folders, but 100 of them are my max cache and are deleted on shutdown.
I see claims for speed everywhere but when I look at the tests it’s a penny change.
This is all stock by the way. By the way, the browser and the entire computer are far from storage, I always try to tweak and reduce everything, and this worked even better with Firefox.
Switching between tabs can be faster in my case, I have most of all open tabs and so I can’t see any if the browser isn’t on top.
So it makes it slower because I can now see all of my windows directly without having to open the browser first.
I also never risk opening something to a stream or sharing a desktop that others aren’t allowed to see.
In terms of synchronization it can be. I do not have this need. I keep my phones and tablets completely separate from my computer, even on my network, they are isolated from each other. All my tablets and phones are syncing. I have Chrome on tablets and phones because the tabs there work better like Firefox with an annoying tab window, on top of that, chrome is also there with lots of snippets and other things that you can’t turn off since last year’s releases because it intentionally used the command line. (Just like many other command lines).
The amount of chromium you use depends largely on the content that you open it up. The broadcast board is really a disaster in my experience. There seems to be a lot of buffering.
And yes, I have also read that there have been many improvements but not enough yet, and for me, tabs and browser cleanliness are basically the deciding factors. (Most of my other programs are portable independently. They work better and give a more modular feel, which gives me flexibility.