I tried your method to install uBlock in palemoon 28. However, I still have a problem.
I don't know where this is but I did manage to download ublock.xpi to "Downloads". On the extentions page there is a small box with what looks like sliders on it next to "search". I clicked on this and one of the options is to "install Add-on from File". This I did but it failed with the output "This add-on could not be installed because palemoon does not support web-extensions. I had the same output when I tried before. If you can point me in the direction of my palemoons profile I'll try again using the method you outlined.Copy it in the extensions folder of your palemoon's profile.
Regards,
Ken.