Hot tip. You can enable 'Install extension from file' in mobile Firefox apps (Firefox, Fennec, Mull etc) by going to Settings > About Firefox (or About Fennec etc) > Tapping the name 5 times.
You should see a message about debug mode being enabled and the 'Install extention from file' option should be in the Advanced section of your brower settings.
I've blocked the .ru domain because Russia invaded Ukraine and is killing their people. Is there an alternate domain that isn't supported by an authoritarian regime killing its neighbors?
Maybe, I have to admit that I never got used to reader mode and I was used to just getting the content seamlessly most of the time. If that works as well that's also a great solution!