Bug 1756450 is a problem where a failure to create the update directory causes exceptions to be thrown and
about:preferences to not be rendered properly. I intend to attach a patch to that bug that will improve the situation by catching this error and printing a generic error message in the Update UI. But ideally we would do better than this.
In the ideal case, we would try to fix bad permissions in order to make sure that we can create the update directory. This might be worth doing, but it can't fully solve the problem because we can't be sure that we have the necessary privileges to change those permissions. In some situations it may be possible to elevate our privileges with the Maintenance Service, but (a) there are situations were we will not be able to do that, and (b) safely changing permissions from privileged processes is very hard.
So regardless of whether or not we ever want to try to fix the permissions, it would probably be good to have a SUMO article that describes known ways of encountering this error message and how to fix them. It would also be nice to encourage the user to file a bug if they are getting the error for a different reason.
Ideally, we would link to this SUMO article from the error message, though I'm not sure how this ought to look. I would really like to include the manual update link as well, and I'm not entirely sure how to include both links in that message and have the message still look good.