I’m using Mullvad VPN. Mullvad recommends using the encrypted dns only when not connected to the vpn. The new Browser comes with DOH enabled by default. Am I supposed to disable it in Network settings or am I supposed to connect Proxy?
Is it “ok” to use “safer mode” and modify ublock origin for example to use medium mode or does this make me stand out more?
Unless you have a special use case I would recommend you just leave all the settings the way they are. The point of the browser like the tor browser is to have multiple people using the same browser with the same settings so that it’s much harder to identify individual users. Just use the browser as is with a vpn connected and you should be fine unless you have a specific reason to change something.
Use the browser on default settings as much as possible. If you want to customize, I’d use Firefox with Arkenfox or Librewolf. Leaving the settings well enough alone on Tor and Mullvad Browser is your best bet for blending into the crowd. So far, Mullvad doesn’t have the user base to blend in yet based on my tests, whereas Tor definitely blends in when used on default settings or the “Safer” setting. My hope is enough people use Mullvad that it becomes similar.
That’s said, if blending in on fingerprinting is not your absolute top priority, tweaking what you said isn’t a huge deal.
Not sure about proxy cos I use ProtonVPN. As for using safer mode, in general if you disable js you won’t have a unique fingerprint. So I guess you can disable js by default on uBlock and enable on a site by site basis, or go for a more granular blocking through NoScript. Just don’t modify any settings in about:config or modify any settings in NoScript apart from enabling/disabling javascript (by other settings I mean font, webgl etc). This is what I did and seens ti be fine. I’m no expert though…just a privacy conscious dude…so best to wait till someone better replies before doing anything 
Either disabling DoH in the Settings or using the proxy is fine.
So far, Mullvad doesn’t have the user base to blend in yet based on my tests
Is this still the same? Or, how can I test this by myself?
On the contrary, most people don’t disable javascript, therefore doing it makes you stand out
There is not a really foolproof way to test this, but some websites accidentally provide little clues. For example, on fingerprint.com, they give you a Visitor ID based on some fairly basic (but not completely naive) fingerprinting metrics. If you use Tor in Whonix, even if you’ve never been to the site, it will often tell you that it’s your third, fourth, or fifth visit. This is because there are lots of other people out there using Tor in Whonix and going to fingerprint.com. The website is fooled into thinking the original visitor came back four times when really it was four different people all using Tor on Whonix. I have never seen this happen with Mullvad. In other words, fingerprint.com always thinks it’s my first visit because no one else with my exact fingerprint has visited in the past two weeks (or however long they keep records before resetting everything). Part of this is likely because it is not as good at standardizing all possible variables like Whonix can, but part of it is also likely that the number of users of Mullvad browser is very low compared to Tor.
The bottom line here is that Mullvad is the best you can get for blending in while resisting fingerprinting on the clearweb, but it’s not on the same level as Tor on Whonix.