With the Purism philosophy of “the power of defaults”, it may be a difficult sell that these extensions should not be installed by default, particularly as it is now straightforward for the owner of the device to uninstall the packages.
There is however always going to be a diversity of opinions about the dividing line between “defaults” and “bloat”.
This is not the recommended approach. However if you frequently blow away your entire disk contents and reflash then it won’t really matter.
Agreed, however I was trying to bring the discussion beyond calls that that “purism should do something about it”, to at least understanding what is happening and empowering the user to handle their situation (though they appear overwhelmed at all that needs to be learned at this point in time… Have to take it step by step, @Ick 
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Agreed, uninstalling the packages is the better approach in this case.
Right, but I am more concerned about the message that Purism is sending to new PureOS users if this issue remains unaddressed.
Agreed. Freedom of choise is what Purism always advertises.
This should also apply for plug-ins and/or extensions.
While I disagree that the current situation is bad – after all, you own your device and can change it using sudo and remove packages, you have that freedom and choice.
I did some thinking and concluded that it IS possible to do what is proposed here, and if anyone believes it strongly they should address it (at the source - Debian, who provide these webext packages…)
Case in point, the browserpass extension requires a “native client”, some kind of software that communicates between the extension and the command line pass util.
Their Makefile has a few options, namely to install it for all users or for the user (and I believe, though not sure, it applies for all profiles of a user). The Makefile symlinks the policies etc files in the appropriate locations to accomplish this.
Would this apply to Firefox extensions as well? I’m unsure. I will not research this further as I am happy with the situation as is (apt remove webext-...), but thought I’d share the knowledge anyway
Who said that the current situation is bad? But it could be a beginning of “we know better what you need” and at some point we also take 2 days to remove bad defaults. For new people it’s may even impossible at some point. Not because they have not the power to control it, but because it becomes more and more complicated to do so.
It starts with an addon here and an addon there and some time later in worst case you have a filter deep into system that blocks internet traffic systeme wide for privacy and security purpose - but with many issues (like website that does not send data to users with filters). You still own and control your system, but you may have troubles to follow 10 steps to uninstall that may also break things if done in a wrong way. PureOS is also made for non-tech-people.
Yes, we’ve seen that before i.e. sudo apt remove onepackage and getting prompted with: this command will also remove “list of seemingly important packages”, and user won’t be sure whether to go ahead, and things might break if the user does go ahead, and then user won’t have sufficient knowledge to fix up the mess.
However ultimately your post is a “slippery slope” argument. The slope may or may not be slippery.
It is also getting away from what the OP actually asked … which was for easier removal, rather than that those extensions are not installed at all.
In my opinion, I would lay down solid markers for what Purism should pre-install or use as default settings e.g.
- and with an easy way of changing.
Ideally this would be covered by documentation e.g. if Purism wants to install a firewall by default then this should be documented and the appropriate command for getting rid of it should be documented. It might be that we end up writing that documentation. 
But we are not all going to agree on what should be pre-installed. For example, I run Privacy Badger on my big computers anyway so I am happy for Purism to pre-install it on my phone (although I have been using Epiphany more than Firefox anyway).