The excellent Go mobile Bewertet mit 5 von 5 Sternen
I found this to be a fantastic product that allows testing of mobile web experience, and speeds up web browsing when you are in a low bandwidth environment.
After trying a Mozilla xpi file I found this fairly simple to use. It took me a few moments to find the small mobile phone icon which toggles the web extension/add-on on and off. On my screen it located on bottom right hand side of the screen. By default it has a prohibited sign super imposed, but once clicked that’s removed and is surrounded by a green background. If you can’t find this or the add-on bar go to menu->view->toolbars->add-on bar or press CRTL / and away you go.