Note - this guide is for clearnet browsers (i.e. Brave, Librewolf, Mullvad Browser, etc.). I will post a future guide for specifically the Tor Browser in a Whonix DispVM, since doing so is more complicated.
Jump to the tl;dr for the guide.
When browsing the web, it is highly recommended to make your browsing experience as ephemeral as possible. The easiest means is to use a browser that clears all data on exit (cache, cookies, and so on). However, Qubes allows you to go a step further. Not only can you choose an ephemeral browser, but also use it within an ephemeral machine.
This is the beauty of DispVMs. You should do the majority, if not the entirety, of your browsing in DispVMs. That way, if one session gets compromised at the OS level, future sessions won’t be compromised because that machine will be disposed of once that session ends.
When opening a browser in a DispVM, however, it will always open as though this is your first time ever opening the browser. This can have some privacy benefits, as the browser treats you like a new user every time. That being said, using light-mode every time you browse can be miserable, OR having to change it to dark-mode for each new DispVM session can be tedious. Furthermore, changing all of the Brave settings each time you start a new session (which can take a few minutes each time) is certainly unrealistic.
And so, tweaking a few settings or adding essential extensions into the DispVM Template can mitigate this process.
tl;dr
- Shut down all existing DispVMs.
- Open the desired browser in the Disposable Template. (e.g. Menu → Templates → default-dvm → Click-name-of-browser)
- Make modifications, e.g. change theme, add extensions, remove Brave bloatware, etc.
- Close browser.
- Shutdown Disposable Template
- Open browser in DispVM
- Verify
I tested this for Librewolf and Brave installed into the Fedora template (which propagates to the default-dvm template).
P.S. - updates to these browsers might wipe out your modifications. Not a guarantee, since I have not tested every browser. I have not observed this behavior except with the Tor Browser (again, this is a future guide), but - fair warning - if your modifications suddenly disappear one day, it’s probably due to updates. But just repeat these steps and it should be fine.