AFAICT there is no way to set this on a Chromebook though - and since Chrome is embedded into the OS using Firefox is really painful in comparison. I guess I will have to use uBO Lite.
edit: There is apparently a way to set it by enabling Developer Mode, but Developer Mode is itself really annoying - it shows a popup screen every boot.
I haven't tried this, but you should also be able to apply enterprise policies with Chrome Enterprise Core[1]. As if you are centrally managing a bunch of Clients, but it's only your one Chromebook of course.
When you open/run a .reg file, it updates your registry, usually preceeded by a warning.
Adding the text below to a text file, saving and running it as a .reg will create and add a value of 2 to "ExtensionManifestV2Availability" in the HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome key
Alternatively, you could do this manually by pressing the Windows key, type "run" (without the quotes) and enter, type "regedit" (without the quotes) and enter, then navigate as far as you can to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome key
You may find there is no "Chrome" key and will need to create it, as well as creating ExtensionManifestV2Availability
You need a Google Workspace account - personal doesn't work. Which is admittedly only $6/month on the starter plan, but signing up just to get uBO working seems a bit much.
I've been running ad blockers for 15 years now at least, but I seem to recall really annoying pop-ups being a part of Internet advertising too, and they happen more than once per boot.
It's true that in normal use you don't reboot a Chromebook that often, and developer mode offers other advantages as well. And apparently it is possible to "neuter" the screen https://web.archive.org/web/20170711043202/https://johnlewis.... Probably the only real solution long-term...
edit: There is apparently a way to set it by enabling Developer Mode, but Developer Mode is itself really annoying - it shows a popup screen every boot.