https://addons.mozilla.org/en-US/firefox/addon/i2p-in-private-browsing/
If you have an I2P node running in the background, you can use this to have both I2P tabs and normal internet tabs open at the same time, this addon relies on firefox's container system (and private tabs) to accomplish this.
If you don't know what I2P is...very roughly, imagine encrypted torrents as a protocol for the entire internet, instead of just torrents.
I2P works (again, roughly) by everyone on it running a local node, and traffic is bundled into amalgamated, anonymized, encrypted packets, and then directed around in a meshnet-like fashion... so you can only read the parts of the packet that are relevant to you and use those to load whatever particular content you are asking for... but also, you're helping pass around data to everyone else.
Functionally, its a very slow, but, imo, pretty secure and private way to have a free VPN, while also essentially making its own sort of 'darknet', in that, like Tor and Onion sites, you cannot actually access the I2P network without I2P... but you can access the clearnet via it, if you configure it to be able to do so.
...
Semi off topic, but:
For general privacy/security... a basic setup would be to also throw on the uBlock Origin, Privacy Badger and Canvas Blocker addons, and also go into your settings and switch to HTTPS Only mode by default, and switch your DNS resolver to NextDNS.
That and basically never allow websites to ask for or get your actual geolocation.
That can all be done in maybe 10 to 20 minutes of setup time, and, while it won't make you perfectly secure or private, will give you a lot of privacy/security bang for your buck, it'll give good scores on:
https://coveryourtracks.eff.org/
... though it will not make you totally immune to fingerprinting nor other more invasive kinds of tracking like pixel tracking... at least some of the latter can I think be helped by setting up an actual seperate email program instead of using browser based email...
..and if you are on linux, you can also use some kind of containerized (at the linux system level, not firefox's internal containers for tabs) approach to run firefox, and also run wayland instead of x11, so that you can have a more sandboxxed environment for the firefox program itself.
Consistently running in a non fingerprintable browser is, at least as far as I know... pretty difficult without spending a considerable amount of time basically just setting up a very customized browser for just that purpose, that will also likely break or confuse a large swath of modern websites that most average people use fairly often...
...maybe someone more knowledgeable on this than me can chime in with critiques/criticism/corrections of what I've described here in this bottom section.